Something that doesn't conflict with the territory covered by any other quest mods. I can't begin to tell you how many quest mods take up the same three or four pieces of turf. And that's while so much else in the way of Morrowind real estate is neglected, too.
I have to agree on this, sadly, with all the different mods out there that add new things to the landscape, it might be hard to ensure that the mod does not conflict with any of them, but at least it might be feasible to check if it conflicts with the most popular ones. If it's a large quest mod, I'd also like the quests to be varied, they shouldn't all just be "Kill X" or "Fetch Y", there should be a little bit more variety than that.
And I also must agree with the suggestion of good writing and multiple choices with meaningful concequences. I want to actually be able to care about the quests and the characters, it shouldn't be something I only do because I want the reward. Also, reward is quite important too. The reward for a quest should be something that makes it worth doing, preferably something unique, as when I go through great trouble to complete a quest, I don't want to come home with an item I could have, and quite possibly already have, acquired through some other means. Of course, the reward should be well balanced to the quest, easier quests should give less valuable rewards, while more difficult ones need something better to feel rewarding.
Also, it would be preferable if the quest includes something for characters of various diffierent levels, perhaps the early quests could be accomplished at low levels, whereas later ones require high levels. With some quest mods, because even the earliest quests need you to be a relatively high level, I often find myself only having original quests I've done many times before to do, at the same time, if a quest mod is designed only with lower level characters in mind, by the time I get around to doing some of the quests from it, they will be extremely easy and the rewards will seem pointless. I'm thinking something along how the main quest is set up, where it's designed so that players will slowly get more powerful as they do it.