As far as the overall community goes, I really do think the Final Fantasy community is the one I prefer. You can pick out obnoxious forums/people for any game, just like you can pick out a very mature community. Everyone thinks the WoW community is horrible, have you ever been to ElitistJerks? They are more mature there than on most forums, they ban you on the spot for making stupid comments or speculations of any sort.

Having played WoW in large PvP environments like raids on faction capitals and 40 vs 40 battlegrounds like Alterac Valley, even 5v5 arena or something like Warsong Gulch, I have never seen, even in a video, jumping causing FPS issues. If you are personally experiencing an FPS drop when people jump in an MMO like WoW, I am amazed that your computer can run FFXIV. If you can find a video that shows someone's FPS meter drop when people start to jump, you are more than welcome to share it with us. As far as using the jump mechanic to exploit targeting in something like PvP, if they do add jump, as long as they implement it correctly that will not even be an issue.