I was determined not to play any MMOs because I didn't want other people in "my" games, but the now-sorely-missed Computer Gaming World included a free trial on the CD that came with the magazine, and I was instantly hooked. Partly because it was such a cool superhero game, but also because (as has been mentioned all over this forum) the players were actually really nice -- adding rather than taking away from the game. I've tried a lot of MMOs since CoH, but nowhere else have I found the combination of a game that suits me so well and a community that is so supportive.
Maybe this leads into an answer to your question, Sentry: my initial fear of PvP in MMOs was that I could never relax, because some other player might be out to gank me at any time. Obviously, I have learned since then that good games manage this in various ways, so you always have safe areas. However, as I have learned about MMOs I have come to dislike PvP for another reason: power set "balancing". When the devs have a requirement that any new power set must not over- or under-match any other power set out there so that no one is at an advantage in PvP, I think it strait-jackets how creative the devs can be with new power sets. It seemed to me from complaints on the forums that the CoH devs did not spend a lot of time balancing powers for PvP, and I think that's why there was such an interesting variety of powers out there. In other MMOs that take PvP more seriously, all the power sets seem kinda dull and vanilla because the spark has been balanced out of them, with every set having pretty much the same powers with only cosmetic variations. IMO, of course.