It's not so much about cutting the game shorter as it is about extending the game in the right places. The leveling process is great for single players as it progresses with the story. MMO's just don't work that way. You play through the story and it ends, and then you either have to spread out the story to the point where you're only getting chunks of it between grinding (FFXI was a grind. If you loved it, fine, but it was masochistic for many, especially those who wanted to play their own way or with their friends who may not have played the most popular jobs. Seriously, trying getting a RDM and WHM together in a party past CoP era.)

This era of gaming cannot be sustained by prolonging the content. You've got to fill in the gaps. If you do fill in the gaps, leveling is going to be comparatively faster.

Right now, Leves were the primary source of leveling, you add a good number of quests and storyline quests to that and of course your leveling pace is going to go up. The point is making content that supports it throughout.

The biggest crux of the matter is top-heavy populations. The slower you make leveling, the more that becomes a problem. Suddenly you are either being handheld through party content by your high level friends or you're stuck feeing alone grinding up to get the levels you need to play at endgame with the rest of your buddies.

There's simply no reason to go back and help as the rewards won't assist endgame goals at all, and any loot drops lose effectiveness as you level anyways. So, better to have a richer endgame and an expedient path to it. Plenty of other ways of getting your grind-feels, and plenty of jobs to level to get your feeling of long term progression.

Leveling one class too slow for you? Go for your Paragon's crown. Or go level a craft or 3 to 50, believe me, you won't be complaining about leveling too fast once you step out of the bubble of combat classes.