I'd just like to mention one element which I think hasn't been touched on very much so far: DM advice.
No matter how good your adventures are, remember that all of them, almost without exception, have to be filtered through DMs. Hence, anything that can help the DM do a better job at bringing out the interesting aspects of an encounter or an adventure will have a significant effect on how the adventure is received.
One good example was the issue mentioned by
mshea and
Mustrum_Ridcully of making it very clear, perhaps in a separate section, how elements of the adventure (encounters, locations, NPCs, items, information, etc.) link to another, e.g. how items or information gained from one encounter could be useful in another. Ideally, this information should be presented both when the element is first encountered and when it is eventually used so that it is easier for the DM to cross-reference.
Similarly, in order to counter the problem mentioned by some of the other posters that encounters are dull, repetitive or not meaningful, you could provide the following information and advice to the DM:
1. What is at stake in this encounter (beyond PC survival)? What happens if the PCs fail to overcome it? How can this information be communicated to the PCs?
2. What is distinctive or interesting about this encounter that would make it stand out from the other encounters in the adventure? How can the DM bring out or emphasize the distinctive or interesting elements? If it is a monster's special ability, how should the DM ensure that it is used in a way that maximizes its impact? If it is a terrain element, how can the DM ensure that the PCs are aware of it and deal with it? (Even if that means staying well away from it.)
3. Perhaps more broadly, what is the purpose of this encounter? What sort of feelings, emotions or thoughts should the DM be trying to evoke in the players, and how should he do it? If it is meant to be a simple encounter for the players to beat down on inferior opponents and feed good about themselves, how should the DM describe the opponents' appearance and reactions? If it is meant to be the climactic encounter of the adventure or campaign arc, how can the DM ensure that tension remains high until the final blow (whichever the winning side happens to be) is struck?
And perhaps the acid test for any encounter should be: if there isn't much at stake, there isn't anything distinctive, and it doesn't have much purpose, then it shouldn't even be in the adventure in the first place!