I thought some could find it interesting which ten sites are the most and least visited for each of the continents. The results are not surprising.

Europe: Most - Paris, Versailles (France), Rome, Venice, Florence (Italy), Westminster, London Tower (UK), Vienna (Austria), Prague (Czech), Grand Place - Brussels (Belgium)
Least - Pico (Portugal), Romuliana (Serbia), Komi Forests, Solovetsky, Ferapontov (Russia), St Kilda (UK), Surtsey (Iceland), Vega (Norway), Angra (Spain), Samos (Greece)
Asia: Most - Great Wall, Imperial Palace, Temple of Heaven, Summer Palace, Imperial Tombs (China), Angkor (Thailand), Taj Mahal, Agra Fort, Red Fort (India), Jerusalem (Israeli controlled)
Least - Putorana, Wrangel (Russia), Uvs Nuur (Mongolia), Sarazm (Tajikistan), Jam (Afghanistan), Rohtas Fort, Takht-i-Bahi (Pakistan), Manas (India), Iwami Ginzan (Japan)
Africa: Most - Pyramids, Cairo, Thebes, Nubian Monuments (Egypt), Tunis, Carthage, Sousse, El Jem, Kairouan (Tunisia), Marrakesh (Morroco)
Least - Loropeni (Burkina Faso), Askia (Mali), Salonga (Dem Congo), Manovo (CAR), Sukur (Nigeria), Algiers, Tipasa, Beni Hammad (Algeria), Kondoa (Tanzania), Tadard Acacus (Lybia)
North and Mesoamerica: Most - Statue of Liberty, Grand Canyon, Independence Hall, Yosemite, Everglades (USA), Quebec, Canadian Rockies (Canada), Tikal (Guatemala), Chichen Itza, Mexico City (Mexico)
Least - Cocos (Costa Rica), Coiba, Darien (Panama), Nahanni, Joggins, Wood Buffalo Park, SGang Gwaay (Canada), Sierra de San Francisco, Yagul and Mitla, El Vizcaino (Mexico)
South America: Most - Machu Picchu, Lima, Cuzco, Arequipa, Nazca (Peru), Iguazu (Brazil), Iguacu, Los Glaciares (Argentina), Quito (Ecuador), Colonia (Uruguay)
Least - Los Katios, Malpelo (Colombia), Rio Abiseo, Caral (Peru), Noel Kampff (Bolivia), Serra de Capivara, Cerrado, Sao Francisco, Atlantic Islands (Brazil), Central Suriname Reserve (Suriname)
Oceania: Most - Sydney Opera House, Great Barrier Reef, Greater Blue Mountains, Wet Tropics of Queensland, Uluru, Royal Exhibition Building, Fraser (Australia), Hawaii Volcanoes (USA), Te Wahipounamu, Tongariro (NZ)
Least - Phoenix (Kiribati), Bikini (Marshall), Papahanaumokuakea (USA), Roi Mata (Vanuato), Kuk (PNG), Lorentz (Indonesia), East Rennel (Solomon), Heard and McDonald (Australia), Macquarie, Subantarctic Islands (NZ)

Surprised not to see the Socotra Archipelago in the list. I only became aware of it a few months ago and am now dreaming for an opportunity to visit!

I´ve just checked the list to see if there are any changes due to the new additions. There are. I´ve just updated the list of least visited sites for each continent.

Europe: Bukovinian Residence (Ukraine), Pico (Portugal), Komi Forests (Russia), St Kilda (UK), Romuliana (Serbia), Solovetsky (Russia), Vega (Norway), Ferapontov (Russia), Surtsey (Iceland), Studenica (Serbia)

Asia: Mongolian Altai (Mongolia), Ho Citadel (Viet Nam), Hiraizumi (Japan), Ogazawara (Japan), Putorana (Russia), Danxia (China), Ardabil (Iran), Iwami Ginzan (Japan), Wrangel (Russia), Ashur (Iraq)

Africa: Loropeni (Burkina Faso), Askia (Mali), W (Niger), Air and Tenere (Niger), Salonga (Congo DR), Manovo Gounda (CAR), Sukur (Nigeria), Dja (Cameroon), Kondoa (Tanzania), Tadrart Acacus (Lybia)

North and Mesoamerica (including Caribbean): Miguasha (Canada), Nahanni (Canada), Cocos (Costa Rica), Coiba (Panama), Sierra de San Francisco (Mexico), Rio Platano (Honduras), SGang Gwaay (Canada), Wood Buffalo (Canada), Darien (Panama), Ilulissat (Denmark)

South America: Los Katios (Colombia), Noel Kempff (Bolivia), Serra da Capivara (Brazil), Rio Abiseo (Peru), Malpelo (Colombia), Caral (Peru), Suriname Reserve (Suriname), Atlantic Islands (Brazil), St. Francisco (Brazil), St. Cruz de Mompox (Colombia)

Oceania: Phoenix (Kiribati), Papahanaumokuakea (USA), Kuk (PNG), Lorentz (Indonesia), Sub Antarctic Islands (NZ), East Rennel (Solomons), Macquarie (NZ), Heard & McDonald (Australia), Bikini (Marshalls), Henderson (UK)

The "Common Factors" for least visited sites? (All continents out of 10 and total out of 60)
a. Natural Sites - 24 + 2 mixed
Europe - 2
Africa - 2
N + Meso America - 8
South America - 5 + 1 mixed
Oceania - 7 + 1 mixed

b. "Smallish" Islands (i.e excl Greenland + New Guinea which have another 3) - 18
Europe - 5
Asia - 1
Africa - 0
N + Meso America - 2
S America - 2
Oceania - 8 (of course!)

c. "Former USSR" (Eur/Asia) - 6

d. "Recently" inscribed -13
2011 - 5
2010 - 4
2009 - 2
2008 - 2

Nice conclusions!

Of course there are also regional tendencies:

Sub Saharan Africa is less frequented then the Arab part (10 to 1 ratio), the southern part of South America (Southern Brazil, Uruguay, Argentina, Chile, Paraguay and Southern Bolivia) is more frequented than its north (0 to 10 ratio), Australia proper is the most frequented part of Oceania (0 to 10 ratio).

Just updated the list of least visited sites. Not much has changed at all during the past year, only few slight changes.

Europe: Pico (Portugal), Komi (Russia), Solovetsky (Russia), Angra (Portugal), Romuliana (Serbia), St. Kilda (UK), Vega (Norway), Ferapontov (Russia), Surtsey (Iceland), Chernivtsi (Urkaine).

Asia: Wrangel (Russia), Manas (India), Putorana (Russia), Sikhote Alin (Russia), Soltaniyeh (Iran), Tamgaly (Kazakhstan), Rohtas (Pakistan), Ogasawara (Japan), Mongolian Altai (Mongolia), Uvs Nuur (Mongolia/Russia).

Africa: Loropeni (Burkina Faso), Air (Niger), Salonga (DR Congo), Manovo (CAR), Sukur (Nigeria), Dja (Cameroon), Tadrart Acacus (Lybia), Kahuzi (DR Congo), Timgad (Algeria), Okapi (DR Congo).

North and Central America: Miguasha (Canada), Cocos (Costa Rica), Nahanni (Canada), SGang Gwaay (Canada), Joggins (Canada), Rio Platano (Honduras), Wood Buffalo (Canada), Darien (Panama), Sierra San Francisco (Mexico), Paquime (Mexico).

South America: Malpelo (Colombia), Rio Abiseo (Peru), Los Katios (Colombia), Noel Kempff (Bolivia), Serra Capivara (Brazil), Central Suriname (Suriname), Sao Francisco (Brazil), Atlantic Forest (Brazil), Discovery Coast (Brazil), Caral (Peru).

Oceania: Phoenix (Kiribati), Papahanaumohuakea (USA), Kuk (PNG), Lorentz (Indonesia), Heard McDonald (Australia), Sub-Antarctic (NZ), East Rennel (Solomon), Macquarie (NZ), Henderson (UK), Bikini (Marshall).

Yet another update:


Least visited sites by continent:

Europe: Chauve Pont d'Arc (France), Chersonese (Ukraine), Komi Forests (Russia), Bolgar (Russia), Surtsey (Iceland), Pico (Portugal), St. Kilda (UK), Solovetsky (Russia), Corvey (Germany), Vineyards of Piedmont (Italy).

Asia: Burnt City (Iran), Tajik NP (Tajikistan), Wrangel (Russia), Hamiguitan (Philippines), Lena Pillars (Russia), Great Himalayan NP (India), Queen's Stepwell (India), Putorana (Russia), Uvs Nuur (Mongolia, Russia), Jam (Afghanistan).

Africa: Sangha Trinational (R. Congo, CAR, Cameroon), Ounianga (Chad), Salonga (DR Congo), Manovo-Gounda (CAR), Garamba (DR Congo), Dja (Senegal), Sukur (Nigeria), Okapi (DR Congo), Kauhzi-Biega (DR Congo), Bassari (Senegal).

North and Central America: El Pinacate (Mexico), Cocos (Costa Rica), Diquis (Costa Rica), Nahanni (Canada), Poverty Point (USA), Darien (Panama), SGang Gwaay (Canada), Rio Platano (Honduras), Red Bay Basque Whaling Station (Canada), Coiba (Panama).

South America: Rio Abiseo (Peru), Malpelo (Colobmbia), Serra da Capivara (Brazil), Los Katios (Colombia), Central Suriname Reserve (Suriname), Noel Kempff (Bolivia), Cerrado (Brazil), Goias (Brazil), Discovery Coast (Brazil), Santa Cruz de Mompox (Colombia).

Oceania: Phoenix (Kiribati), Kuk (PNG), Lorentz (Indonesia), Heard and McDonald (Australia), East Rennell (Solomons), Papahanaumokuakea (USA), Sub-Antarctic Islands (NZ), Ogasawara (Japan), Henderson (UK), Roi Mata (Vanuato).

I am now checking Solivagant's observations from two years ago:

A) Natural sites: 36+4 mixed (out of 60 - 66%) - up from previous 24.
Europe: 2+1 mixed
Asia: 7
Africa: 8
North and Central America: 6
South America: 6+2 mixed
Oceania: 7+1 mixed

B) Islands (excluding New Guinea): 18
Europe: 4
Asia: 2
Africa: 0
N and C America: 3
S America: 1
Oceania: 8

C) Former USSR: 8 (up from 6)

D) Inscribed during the last 5 years: 22 (out of 60 - about 36%)

2014: 9
2013: 4
2012: 4
2011: 1
2010: 4