The problem is that these two parts of the game [Solo and Group content] are drifting further and further apart in World of Warcraft and the MMORPG genre in general. Soloing becomes easier and easier, the need to group during leveling up has been nearly completely removed, elite mobs turned into soloable non-elites, and the rewards for pseudo-solo PvP have been much increased. It is now possible to go from level 1 to level 70 and full epic gear in World of Warcraft without ever joining a group once. And the classes who are best at soloing fast or best at PvP are the most popular and most played.
Meanwhile, raiding remains hard, because that is the very reason of being for it, and even harder raid content as added to the end with every content patch. But to overcome these challenges, people need to learn how to play in a coordinated way. And the mix of classes, talents, and gear required for raiding is very different from what is most popular and easy to achieve in the soloing part of the game. Slowly but surely the two modes of gameplay drift so far apart that cracks begin to appear, threatening the whole model.
From a raider's point of view the leveling game now fails to fulfill it's function of getting people ready to raid. Sure, they might be level 70 and have epic gear, but they might still be totally useless for a raid: they have not even the most basic training of how to play their class in a group, and they are of the wrong class, wrong spec, and wearing gear with the wrong bonuses to succeed in raids...The average player who soloed up to 70, invested some effort in PvP to get epic gear, and now wants to raid, will find himself rejected and laughed at by the top raiding guilds on his server. He'll complain about them being elitist, but in fact it is game design that created the gap between average player and raider.
What needs to be done is to rethink the concept of solofication. Why is soloing popular? A part of it is due to Real Life ® contraints, if you solo you can play in smaller bits and bites, group play needs longer periods. But another part of it is just a Skinner box: people like soloing because the game teaches them that soloing is the easiest way to advance. So even if they would have the time for a group, they rather keep on playing solo, because setting up a group is so not worth it... And WoW's concept of teaching players how to group is equivalent of throwing them into deep water to teach him how to swim: some people learn it that way, but many get hurt and frustrated in the process.
Solofication not only opens up a gap to end game raid content, it also moves MMORPGs in a direction where they become vulnerable to competition from single-player games. When I recently asked whether people would play a single-player version of WoW without monthly fees, I was surprised of how many people would prefer such a game over an online MMORPG with monthly fees. If game design minimizes your interaction with other players, then why pay $15 a month for that interaction?
I have to admit: I prefer the 'forced grouping style' MMORPG. I would rather not solo most of my time -- I started playing MMOs because of the social aspects, not in spite of it. Five and 10-man dungeons in WoW are my favorite content. I wish more quests were like that too -- as it is now, it's pretty hard to find groups for the few 'Group' quests out there. Many people skip them. It's rarely worth the time to look for a group when you could just move on to the next solo quest.
The 2 opposite kinds of 'wants' of MMO players can't both be supported equally; one or the other will fail and as a result the whole game will suffer. I think this just goes to show that a MMO should pick its niche and play to it 100% -- trying to court both is not the way to have the best overall content for its players. One or the other side will end up feeling shortchanged, and the entire original purpose for playing an MMORPG in the first place is lost.