As a high rated arena player (6x Gladiator/1X Rank 1 in various seasons of WOW) and having played lot of MMOs I would say it depends if you're into PVE or PVP. Unlike your traditional fps/racing games - MMO PVP requires more skill (perhaps different type of skill set if I'm using the right term) and is far more rewarding as you're playing real people.
Unlike FPS for instance - run,shoot, hide, follow objective etc PVP in MMO is far more demanding IMO. Offcourse this isn't the case in every game. You can play against real people in COD, Battlefield, Counterstrike etc, but I'm just saying in general.
Take WOW for example - You need combination of game knowledge, class knowledge, CD management, fast reflex/judgement, situational awareness, positioning, team synergy, efficient crowd control, astute timing etc to succeed in Arena (3v3). When I played Hunter, I had well over 50 keybinds and I played at the time when they were not highly represented in competitive PVP.
It doesn't really matter now as they have dumbed down the game for casuals. I quit WOW and usually just play on private servers. Other MMO's have bigger class diversity, but they are not as polished as wow. PVP is not competitive. So, yeah.
You can try Blade and soul. Really good combat style, fast paced, competitive 1v1 system.