A plethora of multiplayer features and variety tainted by young kids, boredom, average graphics and the campaign.
But after a while the game just fell inferior to newer and better titles like Call of Duty 4 and Battlefield: Bad Company. The graphics are kind of dated, too. Additionally, after much Call of Duty 4 experience, it just irritates you in how hard it is to kill people. Sure, this game wasn't meant to be realistic but it just isn't fun to play anymore. Lag is also an issue, especially in 4-player campaign, and any large game with 10 or more players suffers from it. Not to mention the unreliability of voice chat and game disconnects (with a black screen).
In the end, Halo 3 does excel in many different aspects, but in the end you'll have more fun with Call of Duty 4.