Personally, I can see both sides of the mandatory argument. I can see how someone who sets great store in raiding feeling that they have to do the dailies to get the epics. They're the best gear available before you actually start raiding proper, and as a result, those among us who want to be as well-prepared as possible feel the need to do them. If you're aiming to take part in a team within a competitive guild, you don't want to be left behind, you feel you have to do the dailies to get the epics. If you're in a guild with raiders you care about, maybe you don't want to be the one holding your raid back because you didn't want to do the dailies.
But on the other hand, as much as I hate saying this, WoW is a game. You don't have to do any part of a game. Heck, you don't even have to play it. The problem I see with this argument is that you want to play it. Well, I do at least! I want to raid, amongst other things, so there is a certain weight I feel to jump through the relevant hoops to prepare myself for the things I want to do. Of course, you can go other ways about it than dailies, in fact I continue to do so. But, every time a guildmate says "you're not doing dailies?" I feel a pang of guilt.
How about you? What do you think? Is there such a thing as mandatory in a game?