This is a hard question to answer, possibly impossible to answer accurately. The problem is that it is incomplete. Let's finish it:
What is fun for you?
Mindless grinds may be your thing. Or you hate them. Maybe you dislike them little enough that you will do them, for achievements, those are what are fun for you.
Challenges perhaps? I'd guess most people like challenges, but usually not so much to create them for themselves. That leads to the next fun.
Gear might be your motivator and excitement. That next drop is what keeps you in the raid.
Ganking? Duels? Arenas? BGs? Mass PvP? Vehicle fights? These may all be your fun.
Questing? Perhaps you like the stories. Maybe you like leveling through quests to build up character lore. Role-playing may be your thing.
I am almost certain that winning and success are part of your fun. But those depend on the goal. Someone may 'waste' an hour camping you, but he is having his fun for he is winning not only at killing you, but he also believes he is frustrating you and that is his goal. On the other hand, you might really enjoy playing cat and mouse, not being frustrated, but instead challenged to see how long you can delay his victories and every second is a point for you. With this you can both win, at least in your minds. But what's wrong with only in your minds? Most of the world is in there anyway, it seems like the logical place to put victory.
It is in our minds that these rectangles in front of us glow not just with images, but with worlds. It is in our minds that we decide what is and is not fun. No outside force can dictate that to us, instead it can only offer possibilities and we may refuse or accept any we wish. This is why WoW is so successful, it allows so many types of fun. Do we wish it had more raids or quests or PvP? Probably. In a way WoW is 75% of a dozen games rather than any one complete game. That means you might rarely find yourself at 100% fun, but for any mood you're likely to find at least 75% and that's far better than you'll find trying to RP in Halo.