This is a topic (group of topics rather) that I've thought quite a bit about.
It was alluded to earlier, but, if you make leveling quick and offer a level 20 dungeon, no one will do it, as the time spent getting a party together & fumbling through the dungeon in a PUG would have been better spent by getting to level 23 and opening up a new set of gear. I think this was one of the truly incredible things FFXI did. It was a completely different sort of gear treadmill.
SWTOR/GW2/WoW introduce an entirely new gear set every few levels, which outclasses your previous set completely. Also, if your leveling quick, why invest in +1 gear, when in 2 levels you can buy nq gear that's better?
This is what FFXI 'cured' by offering items throughout the leveling process that were difficult to obtain, but were certainly status symbols and allowed you to rock that item for months and in some cases, were elite till expansions came out. For example, in the early years, it wasn't uncommon to see lvl 60+ thiefs wearing a emporess hairpin/LLboots and macro in a rabbit charm. Same goes for a WAR/SAM etc. who ground out a pair of sniper rings @ 40. These were status symbols, that you could hang your hat on and be proud of .
By incoporating such items, you give reason to the leveling process. Heck, with expansions, FFXI put even more emphasis on this, with having to go back and get your lvl 50 swift belt. These were great things. Yes it was a pain sometimes, but the next time you leveled a characted to 50 and you had that waiting for you @ the 'ding', it was a great feeling. This is something that I feel all current MMO's lack. There's no attachment to your items, it's simply a place holder for the next 2 levels when the next generic piece of armour will be ready.As opposed to making you work a little harder, but getting a piece of armour that will stick with you for the next 20/30/40 levels (aka atleast in the early going of FFXI 1-6 months).