What's the most annoying thing about Halo 3? People who sit there asking if anyone's up for achievement whoring. That's who.
With Halo 3 being the most popular game Xbox Live is ever likely to see, you're bound to get a mix of players - good and bad.
There's the player who takes it all a bit seriously and actually has tactics for every game type that comes up. His ways of winning are probably written in marker pen (copied off GameFAQs) on his bedroom wall so he can see them at all time. This player is harmless but annoyingly good. He'll also sit there and tell you how many medals he got after the match.