Halo is arguably the most powerful video games IP there is at the moment. It sells millions, it sells systems like nobody's business (people are buying X360s to play Halo 2, believe it or not) and it has more followers (read: fanboys) than any other game.
Is that actually such a good thing?
The Xbox name is generally associated with Halo and not the other most excellent franchises that are on the platform (Ninja Gaiden, Splinter Cell, Crimson Skies, etc. That can easily generate bad publicity and misconception among video gamers (example: Stealdragon here).
Developers and publishers alike are scared of Halo. When Halo 2 hit the shelves, people's interest in other games of the same genre (like Half Life 2) was definitely dissolved. We've recently learned that Unreal Tournament 3 and Haze would be timed exclusives. These are such big titles for the genre lovers. Getting UT3 off Sony's hands for the console exclusivity was a huge thing for Microsoft. But even the Unreal beast and the legendary Epic Games can't fight against Halo 3, hence the delay of the game's release on X360. UT3 won't run on most people's computers easily and getting a $600 console is the easiest, safest bet here. There are more hardcore Unreal Tournament fans than hardcore Halo fans out there and many would be tempted to get a PS3 just to be able to play it the earliest they can. Is Microsoft losing sales of the X360 because of this?