When I played the original Everquest a decade ago, the average time players needed to reach the level cap was 2,000 hours. When World of Warcraft was released in 2004, the average time to reach the level cap in WoW was 500 hours. Today the average is *less* than 500 hours, in spite of the level cap being 80 now instead of 60; a veteran player can level an alt from 1 to 80 in 200 hours without breaking a sweat. But even a new player has an easier time nowadays to reach the level cap, both due to better information from various guides and websites, and due to Blizzard having streamlined many parts of the leveling process.
Now some people believe that faster leveling is always better. Gaining a level is one of the stronger rewards a MMORPG hands out, and people just love getting rewarded frequently. But player levels are linked to level-appropriate content in a persistent, non-random world, and as we can't make infinite content, we can't have infinite levels. There has to be a level cap. And obviously the original Everquest and today's World of Warcraft have very different philosophies about what the purpose of the level cap is. Everquest had a lot of players who never ever reached the level cap, I was one of them. If you don't play a lot, and maybe have some alts on the side, a "2,000 hours to the level cap" requirement can result in the level cap being lifted by an expansion before you hit the cap. It is thus possible to be *always* leveling, and obviously Verant/SOE considered that as a good option: If you never hit the level cap, you never run out of content.
The big disadvantage of such a game with endless leveling is that it seggregates players, mostly by amount of time spent playing, and to a lesser degree by how efficient they are in leveling. And the experience of Everquest showed that the players who reach the level cap do not necessarily consider that as a "Game Over" message, but are quite willing to keep on playing with characters at the level cap, as long as the devs provide something level-cap appropriate for them to do, for example raiding. Thus the different idea of World of Warcraft: If everybody levels up quickly, then most players will be at the level cap for most of the time between two expansions, and will be able to all play together, being all the same level.
Unfortunately that didn't work out that way. Because even at the level cap character development is still important. It just isn't expressed in terms of leveling up any more, but in terms of improving your gear. Some form of reward to improve characters is necessary to keep people playing. According to the Daedalus Project of Nick Yee the average MMO player spends a bit over 20 hours per week in a game, which adds up to 1,000 hours per year, or 2,000 hours between two WoW expansions. Imagine you would spend only 200 hours of those 2,000 hours leveling up your character, and then you'd be completely blocked in development for the other 1,800 hours until the next expansion. Most players would quit long before that. Thus today players reach the level cap, and then switch into a different mode of World of Warcraft, in which they mostly level up their Gearscore in heroic dungeons and raids.
So now we arrived at a truly perverse situation, where people quickly level up to the level cap, but end up being NOT of similar power at that cap. As yesterday's interesting discussion pointed out, today one of the biggest social problems of World of Warcraft is the random Dungeon Finder putting people together into a level 80 heroics group with vastly different levels of gear, skill, and even goals. As I am currently playing a level 76 druid and a level 62 shaman, I made the surprising discovery that a random group assembled by the Dungeon Finder during leveling is *more* homogeneous in terms of power and purpose than a level 80 random group found with the same tool. To boot, "normal" dungeons played through with a level-appropriate group are *more* challenging than the average "heroic" dungeon, because the average group for a heroic is completely overgeared compared to the content.
Some people blame the Dungeon Finder, but in fact that tool works perfectly well to assemble a homogeneous group and provide a reasonable challenge to them in the lower levels. Where it completely breaks down is at the level cap, because the possible difference between two level 80 characters can be so huge. Imagine a freshly dinged level 80 tank in iLevel 187 decent blue gear: There is simply no way the Dungeon Finder can put him into a working heroics group today. His best option is to sign on as dps, because groups are used to at least one dps guy doing basically nothing on any heroic run.
And I'm not sure this will get any better in Cataclysm. Of course at first things will get better, when everybody at level 85 is of similar gear level. But as long as the justice points or other rewards enable players to get the highest level of gear from endlessly running heroics, the seggregation of players will continue in the next expansion. Maybe the Dungeon Finder has to be reprogrammed to put groups of similar Gearscore together and then upscale the monsters in heroics accordingly so that the challenge remains the same for every group. But then what's the point of becoming stronger if the content isn't getting easier by that? Maybe a game which took a bit longer to reach the level cap would be better after all.