It has been mathematically proven that DDO has a significant lack of quests for the mid-teens (the game's level cap is 20). You might think that Free to Play is the model that would solve this problem, as there would be a market for content to fill that gap. Instead, the most recent adventure packs have focused on level 5, levels 6-8, and soon levels 9-12. What's keeping them from addressing the more pressing level range?
The issue is one of numbers. There's always going to need to be some attention to endgame, because players at max level have nothing to do if you're not adding endgame. There's always going to need to be some attention to the early game because new players and alts have to get through that range, and might give up if the content is not good enough. This leaves an unfortunate neglected middle ground that's just temporary enough that the developers don't feel they can spend too much time on it, and just high enough into the game that it isn't really going to be a make-or-break selling point.
For example, I don't have any characters above level 6 at the moment, so a new level 15 adventure pack isn't going to be driving my purchasing decisions in DDO for the time being. When I do get up that high, I might start thinking about trying to save my Turbine Points for endgame content, rather than spending them on a level range that I might not make it back to a second time.
Coping with the middle-game
Every game has this sort of murky middle ground. Often, it'll be found towards the upper end of the levels that were available when the game launched (e.g. 40-58 in pre-Cataclysm WoW, the 40-68 range that SOE's been slowly working on in EQ2, 30-50 in LOTRO). My gut says that the challenges may actually be worse under the DDO model because the game's income is so directly tied to whether people want to pay money for the new content. In response, Turbine has been trying to have it both ways by implementing content that is designed for low level characters and then offers a scaled up version for endgame players, with the middle game left as the odd level range out.
Interestingly, a solution to this problem for the DDO model might be lurking in one of the game's least well-regarded adventure packs, the Devil Assault. This pack gets raised eyebrows because it consists of a single quest, which sounds vaguely like WoW's Violet Hold, with the player standing still in a room defending it against attackers. The thing that makes this quest unique is the level range options. Typically, the optional harder difficulty settings on DDO quests bump the level by 1 or 2 above normal. In Devil Assault, the levels jump from 6 to 12 to 18.
If Turbine is really struggling to make one content pack fit all level ranges, they should consider using this approach with future adventure packs; have a low level option and a level 20 epic option, as they do currently, but add in an additional option in the mid levels. DDO quests are worth less exp each time they are repeated (with the exception that you get the full award for the first time you complete the quest on each difficulty level), so there's relatively limited danger that players will be able to ride a handful of adventure packs all the way to the cap with this approach. Meanwhile, it would make the new packs attractive in a way that yet another addition to the already crowded 1-8 level range simply is not these days.
Cataclysm Post-script
The quirk to Blizzard's decision to spend so much of WoW's new expansion working on the previously neglected level range is that they're really not going to have much of a hole left in the leveling game by the time they're done. Some people are adamant that the TBC-era content from 58-68 is markedly worse than the Wrath era content from 68-80, but the difference in quality is FAR less than the difference between sparse zones like Azshara and Felwood in the 40's and Outland in the 60's.
Ironically, if Wrath is any indication, WoW's neglected middle ground post-Cataclysm may be for non-raiders at the level cap. If you're actually looking to raid, it sounds like the options will be comparable to what we've seen in the past. By contrast, there will only be 8 five-man dungeons at Cataclysm's launch, down from the twelve that Wrath had. It sounds like Cataclysm will keep the system of bribing overgeared raiders into these instances for daily rewards, so this may once again mean that they're not all that interesting for anyone who regards them as the final destination, rather than a temporary waypoint en route to the raid game.