One of the more memorable experiences in my MMORPG career was being invited into a group of Japanese players in Final Fantasy XI, and ending up playing well together in spite of them not speaking English, and me not speaking Japanese. That episode was based on two features of FFXI: international servers and a very good LFG system. Notwithstanding all the improvements to the MMORPG genre since, I must say that the ease of playing with complete strangers has gone down. Several people mentioned in the discussion of MMORPG dimensions that adding content to a game dilutes the player population, and that finding a group in WoW for the Scarlet Monastery is nearly impossible. Why should that have to be so?
With now 9 million subscribers obviously at any given moment there are at least 5 people around who would want to go to Scarlet Monastery. The problem is to bring them together. One issue is that with the current setup not everybody can play with everybody. You need to be on the same server and in the same faction to be able to form a group. Cross-server instances would solve most of that problem, that is a solution which worked well for battlegrounds. Enabling Horde to group with Alliance would also help, it is not as if the two sides were really fighting each other much any more since TBC, they always seem to be pursueing the exactly same goals.
But the more interesting question from a MMORPG design point of view is why the WoW LFG systems don't work. They've gone through several variations and improvements, and still nobody uses them. Part of the answer is a vicious cycle: Nobody uses them because nobody uses them. If WoW would have *started* with a good LFG system two-and-a-half years ago, using it would have become part of the way the game is played. But as the first variations were so bad, people got trained to ignore the LFG system, and even improvements don't change old habits. The other part of the answer is the "Vision" argument: People don't use the LFG system because soloing is too easy. This is really, really hard to get right. Between the people who always solo and the people who always group there is a large population of players who could play either way, depending on the situation. If it is easy enough to find a group, and being in a group gives you a large enough bonus to experience and loot possibilities, these players will group. If finding a group is too hard, and you don't miss much by not joining one, they will solo.
I think previous design attempts have been too binary: Content is designed to be either solo or group. You can't solo Scarlet Monastery at level 40, and you wouldn't group for doing non-elite level 40 quests in Stranglethorn, because there is no point in doing so. A better solution would be to make all content accessible to both solo and group players, but scale the challenge and reward level with the group size. Then you can fine tune the amount of bonus you need to give for people to be encouraged, but not forced, to group. That still doesn't solve the problem of dilution. But it might get us away from the "solo to level cap, then group" mentality that is currently prevailing.