While I'm often defending the point of view of the casual MMORPG player, I can't honestly say that I'm a real casual player myself. I play too many hours, in too large blocks, I read (and write) too much about these games, and I even participated in raids. But fortunately I have a real example of the species "Ludio Casualis" at home, my wife. Although she plays World of Warcraft since over 2 years, she has never hit the level cap. She played several characters to between level 40 and 50 before she finally discovered that rogue is her preferred character class. She made it to level 60 only after the Burning Crusade was already out. And last weekend she dinged 66 in Nagrand. Even at her speed she'll reach level 70 in a few months, long before the next expansion comes out.
And now she asked me what she should do when she eventually reaches level 70. I didn't have a good response. In the two years she played my wife *never* visited an instance, except one low-level dungeon together with one of my high-level character. She doesn't know the first thing about tanking/healing/dps aggro management in a 5-man group. She never even *was* in a 5-man group, the biggest groups she did was with two other random players killing some elite mob for an elite quest. Most elite quests she just abandons. And of course she never ever did any sort of PvP, she isn't interested at all in that. So if she doesn't want to do instances, and she doesn't want to do PvP, what can she still do at level 70? She can do some remaining quests for the item rewards, even if there are no more xp. Or she could grind some faction. But for all practical purposes the game is over for her. She could start a new character, but she repeatedly tells me that she doesn't know how she ever lived without Vanish, and I don't know what other class she could still have fun with.
There is no rule saying "a casual player plays X hours per week / month", and not everybody started WoW over 2 years ago, so nobody can say how many players are in the same situation. But it is safe to say that a majority of players, however casual, can reach level 70 in the 3+ years from WoW's release to the release of the second expansion. And neither WoW nor any other game of the genre has come up with a good plan on what to do at the level cap if you don't want to do PvP or groups. In spite of the "massively multiplayer" label, it is the ability to play without too much interaction with other players that got World of Warcraft to 8.5 million subscribers. Losing this feature at the level cap risks losing these solo players when they finally reach the cap, and I suspect that there are more of them around than you'd think. They aren't that visible, because they don't write on forums or blogs, and don't interact much with other players in the game. I once called them the dark matter of WoW. But in spite of being invisible, they are having a profound impact on Blizzard's bottom line, because each of them pays the same $15 per month than the most famous guild raid leader. We might not even notice them leaving, except for the fact that we'll wonder one day why Blizzard never announced 10 million subscribers for WoW. The day more people are leaving than coming new into the game, World of Warcraft will start its slow decline. And that day could well be now, or at least not very far away.