This is a good topic. I started taking Greek classes 11 years ago in preparation for moving here and only actually moved here in 2009, when I was fully fluent. However, I did take trips of various lengths (lasting from one week to 12 months) over the course of the 10 years that I was "prepping" for my move. Now that I'm here permanently (and engaged to a local), I am fluent in the language and that has made things MUCH easier (and it's less necessary for me to have connections with other Americans / English-speakers). I think the language issue is the biggest one, and that's why I spent so much time learning the language formally and informally before moving. (Greek is one of the hardest languages to learn, so it takes many years - I wasn't actually expecting it to take that long myself.)
I have American friends (who were my friends in the US before I moved here) who also spend a lot of time in Greece (I have a lot in common with my friends - most of my friends have a very strong connection with the country that I'm an expat in), and I love to spend time with them here in Greece. But I have never become involved with the "expat community" as such in Greece (beyond posting on this website) because I find the complaining to be very annoying, and I just like to live my life without constantly comparing things to how they are "in the States". (However I am guilty of doing this myself when I'm in a really bad mood.)
Yes, here too, a lot of the expats that I have met spend a lot of time and energy complaining about everything (bureaucracy, prices, the media, food, availability of certain products, public transportation, etc). I can understand complaining about things like the exchange rate, since that's very variable, but why would you complain about the food (for example) of a country that YOU chose to move to? Wouldn't you want to come here for a few months before MOVING here to make sure you actually LIKE it here? That really irks me and as a result I completely avoid those people.
I have met some "seasonal expats" (who are only in Greece in the summer), who are a lot less complain-y because they are only here for a few months and don't deal with some of the "daily life issues" that the rest of us either complain about or just deal with - and they are a lot more positive and fun to be around, I often find. On the other hand, they are only here for a few months out of the year so I don't see them as often!