As regular "Frothing at the Mouth" readers know, I have been reading submissions for M-Brane SF magazine as guest editor for issue #12. It has been an interesting experience since until now, I have never slushed. While I haven't received any truly atrocious material like I hoped to read, I have received quite a bit of not-ready-for-prime-time stories.
Now, bear in mind that I am electronically networked to a lot of SF writers, I suspect the material coming in is a little bit more competitive than than what Christopher normally sees. That said, the story I expect will either lead or close the issue is from a writer I had never heard of before. I rejected five stories that I sent back to Chris for consideration in a future issue. In those cases, generally the main problem with the story was that it just didn't resonate with me the way others did. They were perfectly good stories and probably will appear somewhere with minimal to no revision.
As for the rest, the rejects fell into three broad categories.
1) I was not hooked--often because the writer started the story with an info-dump.
2) The prose was sloppy. Not wrong spelling or poor grammar, simply not the best story-telling technique on the microscopic level: adverbs, said-bookisms, and so forth.
3) Nothing of consequence really happened.
For case 1, not hooked, I pretty much stopped reading when the story lost my interest. there isn't much point on spending time with a story after I have decided I don't want it for the issue.
For case 2, sometimes the story pulled me through much farther than a case 1. With the right story, I might still consider it with some rewrites. At this late stage, the competition for a slot is stiff enough that I can fill the issue without having to do rewrite requests.
For case 3, I suspect the writers are still pretty new at this game and not enough of the story inside the writer's head is getting out.