We've seen it happen quite a few times in MMOs, some group gets their entire WoW guild to invest in a game, and they rush to the end. After reaching level cap before then end of the first month, they declare the game boring, as there is nothing to do at level cap. An MMO has more challenges than just creating a fun world to play in: they need to engage the player at the onset of the adventure, allow enough activity for them to survive the mid-level doldrums and they need to have an end-game worth investing time into.
In all three cases, the main ingredient is allowing the player to feel progress. Each level must have meaning, and each piece of gear must allow the player to differentiate himself from who he used to be. Though each level need not have the same rewards, it is important to have something there. With this being the case, perhaps there are other progression systems that can be made available throughout a game, so that there is always some ethereal goal to attain.
World of Warcraft does this in several ways. First there is the leveling game: this is the easiest to implement and most straight forward advancement possible. Next there is a gear's ilvl: at some point you will need to reach a certain threshold of gear to progress, and each piece of gear adds to your survivability or damage throughput. Next is faction advancement: this is currently the more ethereal concept, but the idea has been slowly advancing to commit to factions to gain gear. What is missing from the faction advancement is an alternative reward not tied to the ilvl advancement. WoW also has skill advancement: primary and secondary professions show a sort of advancement as well.
Of course, the problem is that all these systems are finite, and at some point you will reach a point where there is nothing else to do. WoW of course then added achievements, and rated PvP. These choices are slightly similar and different from ilvl and faction advancement, but they serve as a functional extension to those who enjoy them.
Of course, WoW is a six year old MMO, with quite a bit of history and expansion under its belt. So a new MMO surely does not have the amount of content to compete with WoW, even if they manage to squeeze over six different forms of character advancement into their game. If that's the case, what can a game do to slow down progress of level-cap rushing players, and still allow the game to give a feeling of progress? We saw the reaction given by the player base to Final Fantasy's concept of limiting player experience gains. We also have heard how people hated having to get keyed for raid access in WoW. Gating is very much a no go for gamers. The oldest form of gating would be grinding, and that is also something that is looked down upon by a majority of people.
The thing is anything that requires repetition is now a grind. The key is instead making the grind fun. I mean, stomping on goombas is a grind, yet people still have fun doing so (to be fair, koopas are more fun). So since a new MMO doesn't want people at level cap by the first five hours, it is important to find ways to extend game play without making the player feel like they are being held back, and instead make it feel like they are being pushed along.