In no particular order...
1. Fans of the original Xbox came to the 360
2. Most affordable HD console (Especially compared to PS3 launch)
3. Strong exclusives (Most notably Halo and GoW)
4. Offers many popular PC games without the price of a high end PC
5. Huge focus on multiplayer (offline and online)
6. Huge focus as being a multimedia device (plays common movie codecs, DVD, Netflix, etc)
7. Head Start (Not a great start considering most of the launch titles are garbage)
8. Advertising up the ass
9. Everyone knows Microsoft
10. Appeals to core audience







