This afternoon I had the opportunity to see an agent reacting to a query letter. The first sentence in the letter was missing a comma. The comma was clearly needed, but it was easy to miss it as there were several adjectival phrases set off in commas so the one that was missed wasn't as obvious as it might have been. Even so, the agent remarked that if he were feeling overwhelmed with submissions (rather a normal state of being for most agents) or even just grumpy, that missed comma would be enough to make him stop reading.
Moral of the story... make darn sure you've dotted your Is and crossed your Ts in your query letter.
The other comment he had was about how the description of the book was rather passive sounding. It's so hard to condense a novel into a paragraph or two. Figuring out which bits of information absolutely must be included and which can be left for later discovery when every word seems vital is, simply put, gut-wrenching.
At least forewarned is forearmed. Your query better not have any grammar errors or typos -- and if the sentence you find yourself writing is one of those where the punctuation isn't obvious, maybe you need to rewrite it. Probably the same could be said for the first few pages of a manuscript. Obviously, you want the entire manuscript to be the best it can be, but those first few pages really make or break it. You'd hate to lose a contract because you hadn't quite figured out the rules for commas, or what a dangling modifier was.