What elements would make an adventure more exciting than another? What is missing or wrong in those adventures that fall flat? The primary common element I see is linearity and "obligatory" content. Is there anything else?
Speaking only for myself? A lack of depth. Most of the 4E modules I've encountered so far are very shallow. That's not intended as a criticism, per se. It's meant that those modules are in service to a different goal than what I want.
For example, one of the 4E modules in a recent dungeon had a great setup. A giant floating stone head (think Zardoz, I'm sure the author did) is coming crashing to the ground. Several suggestions are made as to why the players might get involved (the wizard who lives there asks for help from atop the head as it is being pulled earthwards, curiosity, greed), but that's the sum total of any kind of story.
The module is basically an excuse to have three or four combats, quickly. Yes, this is meant as more of a side-trek and yes, this is more of a fun idea than a full-fledged story...but it is the extreme example that illustrates the focus I'm unimpressed with. The module's plot is this: follow the head, fight some monsters that attack the head, go inside the head and capture or kill the elementals that power it before they escape and then go find and kill the dudes who forced it out of the sky. The end. The only decision points for the players are: Get Involved or Don't Get Involved? Steal the Head or not?
And in answer to someone's earlier question about what order the players encounter things? Yes, it is important. At least to me. Many modules have played out very differently depending on the players choice of attack or direction. It is, to me, a major player input. Saying that any door they take will
always lead to the trap or any chest they open will
always have the loot they're looking for or the mcguffin? It's not nearly as interesting, to me. I don't want a sandbox, but I DO want an environment. A flowchart adventure isn't a bad thing, though...I LOVE adventures that act as a flowchart. But NOT if the flowchart is a SINGLE LINE, which most 4E adventures feel like.
On a related note: whoever recommended Revenge of the Iron Lich, the fourthcore adventure? Kudos to you, sir or madam. It is KICK. ASS.