I don't know if you've seen this post about the main reasons agents, and editors, often stop reading. It made me look at my manuscript in a whole new way.
Add to that the posts Kristin Nelson is doing about why she will pass even if the writing is good and you've got a great little great little checklist for revisions.
Pacing seems to be a big one. I highly recommend you read these articles in depth. You'll be glad you did.