True, our dear friend The Rejectionist has a point: no matter how heartbreakingly beautiful your prose, you're not going to sell a book if nothing actually happens in it. And, to be brutally honest, you can probably sell a copy or two with a great idea, great pacing, and relatively average writing. However, you cannot, cannot, cannot make errors in spelling, grammar, or punctuation and assume they'll all be caught by editorial before your book goes to print. You can't be stylistically boring. In fact, you can't just be good. You can't even be great. You need to be as perfect as humanly possible.
Here's why: every week in this country, tens of thousands of books are published. Said published books are only a fraction of the books that are accepted by agents and shopped to publishers, which are in turn only a fraction of all the books written and submitted by you, the unpublished. (Much like Editorial Anonymous, I am not a fan of the phrase "pre-published." Litotes, people!)
The point is, there are hundreds of thousands—if not millions—of people competing with you for book deals. Since (sadly) most of you won't get one, you need every advantage possible if you want even a prayer of seeing your work on a book store shelf someday. Now, it might be enough to have a solid plot, great pacing, good voice, strong characters. You might be able to get away with average writing/spelling/grammar if you've got a stellar agent and editor on your hands. Then again, maybe not. Why leave anything to chance?
In short: make sure you dodecatuple-check your MS, get all your !s, ?s, ""s, ;s, and —s squared away, and only send agents the absolute best piece of work you are capable of creating. They're going all out for you. You need to go all out for you, too.