I think the Greek team should have an opinion on that by now.
Well, having organised an UnCon in Skyros for greek bookcrossers I can tell you the differences:
1. You're not bound by the time of the Convention, so it doesn;t have to be around the 21st of April. You can schedule it for the time that best fits your own city.
2. The official language is the language of the hosting city and not English. This makes a difference mostly in the scheduled events and printed material. However even in our greek Uncons, website, programme, maps and everything else was bilingual, eventhough we didn't have any foreign partcipants in the end. Anyway translating everything in English for us greeks was very easy and wouldn't be a problem.
3. We didn't have official bookcrossing store with us in our Greek UnCons, but in two of them we had bookcrossers from abroad attending.The official announcements should be made in more different forums (for different language speakers). The problem with transferring money or making registrations from abroad wouldn't be much of a problem for Athens Convention if we didn't have capital controls in Greece due to the crisis at the moment. Other than that it's easily solved via internet.
4. There are NO OTHER differences . The style of each Con or Uncon is the one of the city or town or people that are organising it. We always have many books, much food, great fun and meet new people.