With today's confirmation of a new reputation system coming to Xbox Live, Microsoft is tacitly acknowledging how miserable it can be to play games against strangers on Xbox Live. It sounds like you'll be subject to how others rate you.
Details are scarce, but we get this much from a letter to Xbox Live gamers by Microsoft's Marc Whitten:
Reputation: Your new Xbox Live Reputation plays a prominent and valuable role rewarding healthy participation while reducing troublemakers and cheaters. A unified system across Xbox Live and all of your games allows for you to have much more control over who you play with while giving you new and exciting ways to get even move involved in your community.
Hopefully that system can't be abused and that well-meaning gamers can be match-made with each other, and jerks with jerks. Cross fingers.