Cameron gives a good ramble on the early levels of Age of Conan in his Player versus Everything column today. Predictably, many of the games shortcomings are typical to MMOs. One mistake, which Cameron mentions, that's made by seemingly all MMOs is a shortage of variables in the early game experience.
Damion Schubert spoke once about how important a player's introductory experience is. The further into the game, the more an MMO player has invested in the game (time, money, friends, work, etc); and so the more likely he is to continue his subscription. Players are most likely to leave in their first month, and are also likely to speak to a friend or on a blog about the game. The introductory experience also sets the tone for the entire game. It shapes how the player will perceive and approach future content.
Damion was talking primarily about quality, but just as important is variation. I'd bet that the vast majority of MMO players either create characters in addition to their main or start over when bored with their main. A significant portion create more than one alternate character, and some players (like me) put a lot of time into those alts. So the early game, more than any other part of the game, must be replayable.
Frankly, the replayability of most MMOs sucks. The main variable is the player's new choice of class and/or race. Games like EQ2, WoW, and CoH offer a choice of starting locales and low-level enemies. And then there's limited choice of gear (limited because there's usually an objectively optimal gearset which most players gravitate to). All of these dynamics are admirable, but they don't add up to a very fresh experience.
Why? The same combat strategy applies to all enemies. The loot is predictable and varies little within an enemy group. Experience points are always rewarded. Weapons and other gear have different numbers, but basically play the same. Locales offer different views, but little else. Nearly all quests are identical in goal and method. All factions work the same way. Every character's basic goals are the same. Level progression, gear progression, area progression, skill progression, faction progression... all progression is completely predictable and generally familiar to the player. There are new elements, but little to be surprised by and get excited about. In short, there's no adventure.
There could be.