I think most of what I love about online gaming has already been covered and applies to almost every genre.
Unpredictable opponents, the sense of accomplishment when you win, the option of hanging out with friends.
Warning: Epic Post Incoming
The problem is that most online games also suffer from the same problem - grind. There's invariably some form of experience system and to increase in level you need to grind (quests, races, kills). Once you level up, you get access to a whole new set of quests, races or kills to grind through.
I appreciate the need for some kind of tiered difficulty, but I don't think you should need to repeat the same actions so often you could do it blindfolded. That's not difficulty, that's an unimaginative delaying system. Events should keep you on your toes and have an element of randomness to them.
Obviously racing games are a little different, and the elements of randomness there come from human error and/or a changing environment (weather conditions or day/night cycles). But those things keep the play dynamic, and that's something I feel is missing from most online games.
I won't ever pay for another subscription-based game (I hate feeling like I SHOULD play a game simply because I've paid for that month), but my ideal online racing sim would have the feel of LFS, the look and customisability of Forza 3 and the dynamic environments of GT5. But taken to the next level so that any track has changing conditions, almost any part can be replaced and it all has a noticeable effect on the drive.
Experience: 6 years of EverQuest, 2 years Motor City Online, 4 years Guild Wars, many rounds of Planetarion, beta tested more MMOs than any human has the right to, average LFS player since 2006.