This is sort of an interesting topic, because on one hand, you have quest which are
unique and which technically have interesting, or at least unorthodox design--Tomb or the Tormented comes to mind. And yet, as far as I know, there are few quests which are as universally despised as that one.
On the other hand, you get quests like Warehouse District or Tear of Dhakaan, which are very vanilla quests, yet everyone always runs them due to great experience. I'd say those are "uninspired," but then by the same token, at least 50% of DDO's quests are equally "uninspired" or "bland." A lot of quests fall in the category of "they're bland, but people still run them anyways," while a lot of others fall in the category of "they're bland and nobody runs them because there's no incentive to do so." I have an inkling that the quests in the latter category, rather than the former, are typically what come to most people's minds first.
There's also the issue that, after playing through any given quests so many dozens of times... the game just gets old. Certainly, you can vary it up by playing through with different builds, but even then, it gradually starts to get stale over time. By that token, I personally kind of feel that every quest is getting bland at this point. I've been trying to get back into DDO over the past week, but I just can't do it; I can play one or two quests but I'm simply too bored to continue. Maybe it's time for me to move on.

If I had to pick any quest as "most bland," though, I would say that all the quests in Sschlindylryn take the cake for me. Not because the quests themselves are terribly boring, mind you, but because the requisite trek through the Underdark is so nightmarishly one-note and stale. Even if I have a somewhat better grasp of the area at this point, the whole are is just fucking depressing and more so than any other area in the game, makes me want to uninstall the game for good and play something else.