I'm late to the thread, but since there were some unanswered questions why, I'd thought I'd lend my perspective. I've been an expat in Switzerland and have bought in the French Riviera in advance of early retirement, using the time in the interim to build relationships and improve language skills.
From the retirement perspective, people who are pursuing this change are looking for new adventures. They often don't have children or are OK with spending only parts of the year with them. Especially in an area with other expats, language is not as large of a barrier and new relationships are actually easier to form than when moving within the US. It does take time to build relationships with locals, often due to language. The most common refrain I see from expats is how busy their social calendar has become. They also have an affinity for the culture and Europe in general. A smaller fraction are driven by societal issues.
Logistically, health care in France is among the highest in the world and will generally cost an early retiree less (Switzerland is even better on quality, I believe). For me, daily living is 20 to 40% less, but comparisons vary depending on where you are living in both countries. (Paris may be more expensive than rural midwestern or southern America, overall. Some things like gas are way more expensive, other things like phone/internet/tv and insurance are fractions of even rural america). Violent and gun crimes are very low, overall. The weather in the Riviera is comparable to coastal central California (but a little more humid in summer). The tax treaty ensures nearly every one is paying no more taxes than if they had stayed in the US. In fact, I've met more than few expats who have switched their initial choices after realizing that the tax burden wiped out cost of living differences with other nearby European destinations (However, there is a wealth tax on real estate, with exclusions and on the level of US property tax rates, and a high inheritance tax). Other downsides include the penchant for strikes and demonstrations (often isolated to major centers), some petty crime in certain city areas, and some political divisions.
Switzerland is generally more expensive and colder but on metrics like health care, civil unrest, crime, quality of life, pollution, economic disparity, political divisions etc are superior to the US (and even its neighbors).
A word on taxes, without renouncing US citizenship (and exit taxes, if they apply), the best you can essentially do (legally) is the same equivalent level as your US federal obligations.