I'll try to answer as best I can, being an MMO player for most of my life.

1. It depends, if you want to do Coil, you'll need a lot of dedication, but you can do a lot without a ton of dedication, like run dungeons, level different jobs and classes, level trade classes and gathering classes - lots of stuff to do, and you can do it casually, it'll just take longer.
2. They're still active.
3. I haven't seen anyone actually mean. Everyone I've run into has been pretty friendly or quiet.
4. Nope, but if you like talking, it can be fun to join one!
5. I don't think there is one, some, like Balmung, is best for roleplayers, since they have a large roleplaying community, but it's also very populated. I'm on Coeurl and it seems to have a population that I'm comfortable with.
6. Originally, my opinion was really negative. I was so used to action combat MMOs and dodging and moving while casting, and my computer was pure crap so I lagged out a lot in this game and eventually quit due to that, blaming it on the game. I recently returned and gave it another chance since I just quit another action combat MMO and I'm getting tired of them for the most part. I like the mechanic-based fights and thinking through with a strategy of what to do and what attacks to use before I have to move. I'm still new to 50 though and working on gearing up, but I'm enjoying it greatly right now.
7. The most fun I've had is just running dungeons with random people, it's fun and you never know who or what you'll get and I've met a lot of really nice people who have been extremely helpful with helping me re-learn some fights in lower level dungeons and just learn higher level ones.