Basically, what it boils down to, I think, is the fact that neither DC nor Marvel really have editors any more.
Guys like Loeb, Azarello, Bendis, JMS and maybe even Winnick all have their strengths, be it dialogue or ideas. Unfortunately, each of them has some very basic problems with pacing and/or story structure.
None of these problems have to be fatal. And, in years past, an editor like Julie Schwartz, Archie Goodwin, Karen Berger or even Jim Shooter would have been there to point out the flaws and send the story back for some "polish" before it went to the artists.
Now, however, editors are more like agents; guys who are more interested in kissing the talent's butt and getting "exclusive contracts" than in actually editing. As a result, these, and other, writers never get that objective third-party review of their scripts before publication.
As a result, we get more and more books where the stories read like intriging first drafts, instead of well-thought-out cohesive works.
