Thailand is massively popular with retired expats from many nations. The majority are from Europe, though there would be more Americans if it was closer. Southeast Asia is about as far from America as you can get.
There is a highly developed tourism infrastructure which can also be accessed by expats. The major areas that retirees settle are Phuket Island, Pattaya, Chiang Mai, Hua Hin, and Bangkok.
The Thai language is difficult to learn for westerners as it uses script characters, and tones are a key aspect of the pronunciation; however English is widely spoken in the areas most settled by expats.
There is a largely clear cut visa program for retirees. You only have to be a mininum of 50 years (a great feature for early retirees), and you don't need to show any kind of pension or social security.
You do need to financially qualify with either a bank account in Thailand, a pension/social security income stream, or both. The total of either or both must be at least 800,000 baht annually (currently about $19,500). You cannot work on this visa. The visa is renewed annually, and the reports are that if you are able to follow the largely clear rules, most people are able to stay on, year after year.
More visa details for the O-A visa:
http://www.thai-la.net/visa/visa-non-im-oa.htm
Americans in Thailand can buy condos in their own name. However, they cannot directly buy houses or land.
The obvious concerns right now include violent incidents in some parts of the southern region of the country (mostly well away from popular areas for expats), concerns about bird flu, and political uncertainty. Most of the major tourist areas that were hard-hit by the tsunami disaster are back in business.