It seems like in almost every level based MMO, you hit a plateau at the mid levels where you don't have any new abilities to look forward to for a while. Up until then new abilities come at a rapid pace. However at some point you've earned all of your core skills, and won't be getting any major new ones for many levels (in some games, not until you start to approach the cap). Making it seem all the worse, the pace of leveling often slows dramatically around the same time.
A lot of MMOs break this up a bit by giving you a mount, the right to buy a house, or some similar tangential increase in character abilities in the middle of it. The mid levels are also generally when the trait/ talent/ specialization system starts to really come into it's own. Despite these distractions, the mid levels are where I am most likely to stall out in an MMO. Recently Allods Online, Age of Conan, and Warhammer Online all lost my interest during this time. Numerous alts in WoW and LoTRO have also stalled out in the mid-levels. Unless I really like the game and class I'm playing, it's a rare character indeed that makes it to the cap.
As of last night, I seem to have hit a mini-doldrum with my Paladin in DDO. Hitting level six was a big power boost. I can now memorize a second spell, I can cure disease, and my spell points more than doubled to 107 (up from 45 at level five). However, looking ahead, it seems that level seven is not going to be a very exciting one. Until I hit level 8, the only new abilities I'll get will be whatever I purchase through specialization points. Even then, the level two spells don't look all that exciting to me. Bull's strength will be nice (essentially +2 to hit and damage in melee). And the similar spell that boosts charisma might be worth using. However, I won't be getting a better healing spell than the one I'm currently using until the ripe old age of 11.
Despite all this, I'm having enough fun that I don't really care for the time being. DDO may prove to be the first MMO since LoTRO that I manage to push through the mid-level doldrums in. More than anything, it will come down to how engaging the mid level content is to someone that is largely a soloist. I'm having an absolute blast with the lowbie stuff I'm doing right now.
Even if the mid-level doldrums stall me out again, the character creation system in DDO is deep enough that I still have a lot of decent fun messing around with alts ahead of me. It's one of the reasons I've bought most of the low level content. I know I like the 1-6 game. Getting to play that game without paying any more from here on out would be far from tragic.