The issue of "National Themes" as mentioned in EnsignYoshi's post http://www.worldheritagesite.org/forums/index.php?action=vthread&forum=8&topic=204
is an interesting one which perhaps deserves a bit of analysis! Whilst these "themes" reflect a genuine reality about each of those countries they also say something interesting about how those countries
a. see (or want to see) themselves
b. wish to project themselves to the outside world.

Below are some statistics about the "themes" adopted by some countries (the exact numbers are open to some adjustment according to subjective assignment of a site to a category)
a. NL – "Water Management" 5 sites out of 7 (5 out of the 6 in continental NL)
b. UK - "Trade, Industry and Science" 10 out of 27 (10 out of 24 in continental UK). Including 7 of the last 8 inscriptions (and also the next 2 likely nominations :- 2009 - Pont-Cysyllte Aqueduct and 2010 another attempt at "Darwin at Downe")
c. China - "Mountain" scenic and cultural areas ("Sacred" or otherwise!) 13 out of 37. Anyone who has been on an excursion in China aimed mainly at locals will know the "Westerner's frustration" as it stops for ages to look and clamber upon some relatively nondescript rocks which have some great cultural resonance for the Chinese
d. Mexico - "Colonial" towns/ensembles 15 out of 29. There is also a theme of Mexico's "fusion culture" as a number of these emphasise the Precolumbian as well as the Hispanic aspects of the sites
e. USA - "Natural" National Parks 13 out of 20. The USA as "Wilderness"!!
f. France - "Christian" (or significantly so) 11 out of 33

Though it is not a National Theme, Berlin tries to sell itself on a modernist/ industrial destination and this was very much reinforced by the amount of time and effort it put into supporting the Berlin Modernist Housing Estates. They had a big campaign, an exhibition at the Bauhaus archive and hosted an international conference on preservation of Modernist architecture.
As I think Solivagant has pointed out before this is very much the way in which the city wishes to promote itself, and the pursuit of a WHS to enhance this was quite notable.

Both Cambodian WHS are Khmer temples, as are most on their tentative list.
Canada has a fossil theme, sort of. 2 fossil cliffs (Joggins and Miguasha) as well as Dinosaur Provincial Park. the Rocky Mountain Parks contain the burgess of shale, a premier Cambrian fossil site, and it is mentioned prominently in the evaluation. the tentative list, also contains some fossil sites.
the Croatian theme of "located around Spit" has been mentioned.
Greece also has a lot of ruins on their list, but is that a theme? or just a statement of the treasures of Greece.

Berlin is also a Unesco city of design (creative cities network), I believe. And that is another feather to its cap as a city known for its modernist architecture.

As for the subject of national themes, yes this is quite interesting. I must just keep in mind that the "national theme" here applies to the nomination, not necessarily to the very history or nature of the country. As several have posted, Greece somewhat has a theme, as well as Cambodia. And I noticed that of course they would because these are centers of unique historic political entities, hence, would naturally have archaeological sites related to their past recognized as WHS. But this can also be a theme considering that the government chose to nominate such places.

As was stated before, in the case of Netherlands, this is how it would like to portray itself, since the struggle between man and the sea/tide has been central to the Dutch identity. As for Great Britain, well the Industrial Revolution started in that country, symbolized by the Ironbridge. Hence, I think they found the money there. This is their bread and butter, and definitely an important part of world history and modern age. Hence, any industrial site in Britain will likely be of outstanding universal value, almost always baking on being the first of its kind. Canada is another interesting example, focusing more on its geological/fossil heritage. With the exception of one (Wood Buffalo NP), all Canadian natural sites made it on the List by meeting criterion viii (geological/fossil significance).

USA, as Solivagant pointed out, is represented by its world-famous national parks. Note that many more world-famous American national parks are not on the List. Same goes for Australia, the other "natural World Heritage giant". Although for Australia, the sites are more varied with respect of ecosystems and values represented. Latin America will almost always have their first inscribed site as a colonial town. China as Solivagant mentioned, favors the scenic mountain areas. And 2 more are in line for 2009 examination. Africa, as a mentioned in a separate post, has increasingly been nominating cultural sites over natural ones. And most of the time, they qualify for criterion vi (intangible heritage).

There is also the frequent (implied) "theme" of "inclusiveness" (or otherwise). Some countries go more out of their way to demonstrate that they are giving due recognition to minorities than others. Mind you the "demonstration" doesn't always reflect the reality of how those minorities are treated!!
I think of
a. Egypt - despite all its potential pharaonic contenders it has found room for 2 Christian sites. But not for any current Coptic sites, though there is a Monasteries site on its T List
b. Iran - was it purely by coincidence that its nomination this year was for a Christian ensemble in this "Islamic republic"?
c. Bangladesh. This Muslim country has inscribed 1 Buddhist site (and has 4 more on its T list) but doesn't seem able to recognise its significant Hindu minority and past eg with the very fine Kantajee Temple or its Zamindar houses e.g. Puthia and Songaon

Another common "theme" is that of "First Peoples". A number of countries have ensured at least a "nod" in that direction – even changing earlier non PC titles eg USA, Canada, New Zealand and Australia

Its hard to define what constitutes an conscious theme, and what is an accidental theme due to the fact that the treasures of the country are all related. The Netherlands is a classic example of the former, many of its treasures have been forsaken, in favour of inscribing sites with a common theme (water and man). Others such as Libya (which might be seen as having a romano-greek ruins theme), or South Korea (half of the sites are/include Temples) are just inscribed because those are the major sites of the country.
The idea of a national theme, its about what face the country wants to present to the world. Often it seems to be characterized by what people believe is unique about them. Thus once something unique is discovered, the more sites inscribed on the list that fit the category, the more people will look at a country and say "oh there's Mexico, it has a wealth of colonial towns. lets go on vacation there".

I don't think Belgium has any themes. I did read once that the Flemish government let experts seek out world heritage worthy entries and they decided to go with the belfries and beguinages first, so a lot of places spread over flanders had a chance to get on the list.

Well I guess that "spreading the jam" around is also a theme and, with the multiple Belfries and Beguinages, Belgium has been quite successful in that!
It is perhaps also interesting to look at what a given country might have inscribed but hasn't - whether on the T List or not even "recognised" within its WHS lists. Now of course that might be because those sites weren't ready for inscription but that also gives an indication of "choices made". A fair number of countries don't have enough inscribed sites to indicate a "theme" but adding their T Lists gives a more rounded picture. Looking at both for Belgium (26 in all) I see the following overlapping themes :-
a. Wealth displayed by innovative private architecture
b. Municipal (as opposed to National) power
c. "War" + defence,
d. "Commerce" and the exchange of ideas,
e. Late medieval Catholic religion.
f. 19th/early 20th century "Golden age"

Well does that encapsulate Belgium?? As already stated on this Forum a country can't really identify what it doesn't have or totally avoid what it does - but it can, I think we would agree, succeed or fail in getting across its main aspects and significant contributions - for instance, the NL list to date does seem rather "unidimensional"!

So (justifiably?), compared, for instance, with the combined inscribed/T lists of some other European countries there is little or nothing for
a. Natural sites (I have seen Hautes Fagernes and couldn't believe it would succeed).
b. Royalty or hereditary aristocracy.
c. Cultural Landscapes
d. Antiquity - a rather "thin" example of Roman remains plus the flint mines

But have they missed getting across some significant aspect of what "Belgium" is/was about within a worldwide context? Perhaps EnsignYoshi can identify sites which have been left off the list which might give another view!

Damn that is a tough question. The problem with belgium is that it's a mishmash of regions. Each having it's own identity and then there's the overarching belgian identity best know in stereotypes like: good food, beer, chocolate, surrealism, comic books, EU-institutions,.... With regards to flanders I think there might be somewhat of a theme being the middle ages and the famous art cities (plantin moretus, belfries, beguinages, bruges,...). Which I think is fitting for Flanders. If I had to choose one thing for flanders it would have been the cathedral of our lady in antwerp, I think that's the most famous symbol for flanders, and it is already included as a belfry on the world heritage list.

Now for belgium, the first thing that comes to mind is the atomium (world fair '58) which might stand a chance, but I'm not sure if it's world heritage worthy. Manneken Pis perhaps, it has a long history and nice story to it, maybe that could make it world heritage worthy though I doubt it...

I doubt the belgian royal palaces stand a change. Maybe the national botanical gardens, I heard they are impressive, but i'm guessing it would be too much like the gardens of Kew.
Antwerp central station and the cinquantainnare in brussels are very lavish buildings, but also build with the money from the belgian congo (bloodstained money I'm guessing)

Personally if something is world heritage worthy I think it should be included on the list.
For instance, Mesa Verde may have little to do with modern america but it's unique and special in the world and just happens to be in the states and so I do think the same thing about world heritage in belgium. If it's special,unique,... it should be included

Having visited quite well Belgium I can say that, considering the TL sites and other sites I have visited, these would have the strongest potential for the inscription:
- Historic Centre of Ghent (one of the most beautiful places I have ever seen, a big gap on the WHL)
- Historic Centre of Bruxelles (including also all the individual sites of Bruxelles on the TL)
- Historic Centre of Antwerpen (perhaps more for its cultural, historical and economic importance and obviously for the Antwerp school with painters such as Rubens, Van Dyck and Jordaens; but probably it would be better to candidate only the most important monumental complexes, because the whole center doesn't have a high degree of authenticity)
- Historic Centre of Leuven (not only the university buildings but the whole centre)
- Buildings by Henry Van de Velde (one of the most important modern architects; serial transboundary candidature with Netherlands and Germany)
- Panorama of the Waterloo Battle (as a serial transboundary candidature with other important panoramas)
- Minerary sites of the Vallonia (maybe a transnational candidature with France including all the Bassin du Centre)
- Maison Guiette (correctly included in the Le Corbusier candidature for next year)
- Palais Stoclet
- Monument of the Great War in the Westhoek (perhaps a transboundary serial nomination with the most important sites of the Great War on the western, eastern and southern front)