I've read several manuscripts lately that have pretty good writing, a good plot, no major mistakes or problems, they've even been submitted perfectly.
So what's the problem?
I just don't care.
When you look at fiction, you basically have two types--plot driven and character driven. You need both to make a good book. What I'm seeing a lot of lately are decent plots without the character development needed to make me care. It's really sad when the main character is about to be eaten by a shark and you find yourself mid-yawn.
Look at your characters. If you didn't know them so well, would you like them? Would you care about them? Will others care about them? If you're not sure, the best thing you can do is find some ruthless readers who don't know you personally. People who know you well will color what they're reading with your personality, they'll "get" your jokes, they'll "hear" your voice. People who don't know you rely on written cues to assess your character's personality and traits. If complete strangers care about your characters, you're probably okay. If they don't, do some rewriting before submitting.