It is interesting that India has just entered the fray regarding an Art Deco inscription with its new T List proposal of "Victorian and Art Deco Ensemble of Mumbai".

We have discussed elsewhere the relative merits of Napier and Miami but the latter will never of course be nominated by USA. So now there are 3 different Art Deco sites on the T List
a. Eritrea - Asmara
b. New Zealand - Napier
c. India - Mumbai
Which will be first? Or are any of them likely to make it?

India's T list description states that the Marine Bay area is "among the largest collection of Art Deco buildings in the world" and Wiki comments that "Mumbai has the second largest number of Art Deco buildings in the world". I note that I have actually stayed in one of the Art Deco hotels along Marine Drive - the good old "Sea Green"! But I wonder whether the concentration in that area is great and distinguished enough - many of Mumbai's Art Deco buildings are spread out elsewhere. And of course there are always India's chronic problems with site management and preservation. Finally, the description of the site shows it to be a rather strange mishmash. Does Victorian Neo-Gothic and Art Deco really coexist satisfactorily in the same nomination?

And what of the other contestants
a. Napier is currently third on NZ's list for putting forward. ( http://www.doc.govt.nz/getting-involved/consultations/results/nz-world-heritage-sites / ). The most recent review carried out of NZs T List potential was pretty confident of its OUV and integrity etc but felt that more work was needed on protection ( http://www.doc.govt.nz/upload/documents/getting-involved/consultations/consultations- results/our-world-heritage/our-world-heritage-2.pdf
b. Asmara has received help from WMF but is struggling with its "Rehabilitation Project" (CARP). This 4 year old document http://www.bk.tudelft.nl/fileadmin/Faculteit/BK/Actueel/Symposia_en_congressen/Africa n_Perpectives/Programme/Built_Heritage/doc/APD_wp_5_gebremedhin_paper.pdf ) describes the assets and the problems. I noted this comment in particular
"CARP had developed some estimates of the level of financial resources required to renovate and preserve a limited number of historical buildings and neighborhoods. The estimate ranged from $50-70 million, an estimate regarded with incredulity. The CARP realized there was very little money for heritage protection any where, even at the World Bank.". Unless UNESCO decides to help Eritrea to preserve Asmara by easing its path to inscription on the basis that more can be achieved with it inscribed than with it not, then there seems little hope!

Is there any particular reason behind this besides the difficulty of US Law regarding nominating historic districts and consent?

Exactly!!
But this is a bit more than a "difficulty" - its reality would seem to be to prevent any area of multiple ownership ever being nominated.

I notice that the detail of the Asmara Link above actually identifies 3 different architectural categories divided into 6 styles
"Eclectism, Modernism with its subset of styles including Novecento, Art Deco, Rationalism and Futurism, and Monumentalism."
And even states
"Despite this popular style there are relatively few examples of Art Deco architecture in Asmara."

Asmara is described therefore as a "Modernist City" encompassing all these styles, and I note that several buildings which, in my ignorance, I have categorised as "Art Deco" are described by the author as "Novecento", "Rationalist" or "Monumentalist". However, as buildings as late as 1937 are still being described as "Art Deco" it can't be merely a matter of Art Deco being "earlier"
Can anyone (E.g Meltwaterfalls) help clarify exactly what makes a building "Art Deco" - is it just the amount of (potentially "unnecessary" in e.g "Rationalist" terms) "decoration". I look at photos of buildings in Napier for instance -e.g The Sound Shell ( https://www.google.co.uk/search?q=sound+shell+napier&hl=en&prmd=imvns&tbm=isch&tbo=u& source=univ&sa=X&ei=vCvcT-nFIsLG8gP28dHGCw&ved=0CHQQsAQ&biw=1366&bih=667 ) and wonder why it isn't "Rationalist" or even "Futurist"!

Good question, and now I'm not 100% sure myself.

I guess I have always associated Art Deco with being closer to Streamline Moderne, lots of strong sweeping lines. But then looking at the Rationalist and Futurist buildings a lot of these things can apply there to, but I had pictured those two as being defined by particular political and artistic schools of thought, both of which would be relevant to Asmara though.

I will have to pick through my books and do a bit of searching to see if I can arrive at any mildly definitive definition.

I hadn't clocked the examples in Mumbai as being Art Deco, but looking at them I am surprised at myself, I liked the buildings but didn't put 2+2 together to think of them as Art Deco. I even went to see a film in one of the prime examples, and had a cocktail in another. I really should pay more attention, but in my defence it was my first days in India it was a bit of a sensory overload.

I had hoped to find a ready made "model" of architectural styles with their time lines and relationships - surely some architectural historian must have compiled such a thing?

Wiki has this which is a bit thin on relationships but a useful list of some "...isms" and their sequence/dates -though it misses out a lot, particularly of the modern ones.
http://en.wikipedia.org/wiki/Timeline_of_architectural_styles

I also found this quite good on e.g Art Deco but is perhaps somewhat "US Oriented"
http://architecture.about.com/od/periodsstyles/Periods_and_Styles.htm
Its summary of Art Deco aspects has helped me differentiate the Art Deco buildings in Asmara from the other "modernist" styles represented there
"Art Deco buildings have many of these features:
Cubic forms
Ziggurat shapes: Terraced pyramid with each story smaller than the one below it
Complex groupings of rectangles or trapezoids
Bands of color
Zigzag designs
Strong sense of line
Illusion of pillars"

Art Deco evolved over time while taking on new forms. The Art Deco buildings of the late 20's and early 30's are often defined by while slightly later structures are quite a bit different. For example early Art Deco buildings in LA, Chicago and New York are very different than the "Art Deco" Buildings in South Beach Miami, in which much of South Beach follows "Streamline Moderne" characteristics.

For the enthusiast there is a great tour of LA Art Deco buildings offered by the LA Conservancy. Cost $20/Duration 2.5 hrs/walking

- I took this tour, and I have visited some of the other worthwile Art Deco buildings in LA county, so I was suprised by the variation of what I saw in Miami.

Yes, categorization of architectural styles is an inexact science - individual buildings are usually going to exhibit a range of influences and are rarely going to be "pure" and archetypal. All the more so in a town or city where buildings will usually have been developed over time, by different architects and rarely to a single all-controlling vision.

I note that this article about Napier's architectural credentials also differentiates between its earlier "Art Deco" forms and South Beach Miami's later "Streamline Moderne" style.
http://awanderingwidow.blogspot.co.uk/2012/05/napier-art-deco-capital-of-world.html

We seem to need at least a 3 tier "model" of styles in order to bring out the main features of different and changing approaches
e.g Category, Style, Sub style as in
Modernist
Art Deco
Streamline Moderne

Comparison with other similar properties (Mumbai)

"Miami Beach has the largest collection of Art Deco architecture in the world. Today it comprises hundreds of hotels, apartments and other structures erected between 1920s and 1940s using Art Deco idioms. The Art Deco stretch located on Miami Beach was designated as a Miami Beach Architectural District in 1979. Though Miami outnumbers the collection of Art Deco buildings in Mumbai, Mumbai can claim to have the second largest collection of Art Deco buildings in the world, well protected under a heritage regulation."

Interesting find, it sure makes Miami Beach sound exceptional.

Montevideo (Uruguay) also added its Art Deco heritage to the Uruguayan T-list. I wonder how it would fair compared with the other ones.