I feel a major need to rant here...
I felt very disappointed at the stupidity of SEGA making Virtual On Marz on PS2 instead of XBox, at least XBox would have handled the game in the way that the Dreamcast handled VOOT and probably had a real Co-op mode, system link mode, XBL and better load times.
I felt disapponted at the lack of more SEGA games on XBox like the VF series, Fighting Viper series, basically I want more SEGA games on XBox, specially a Panzer Dragoon Saga version or sequel.
Doom III was not really a let down because I knew that RARE and Bungie and Valve have always made better videogames than John Carmack's Id Software, so basically I am disappointed at the engine being that it relies too much on CPU power and was made to purposely run slow doing shadows if you have a slow CPU to force you to upgrade.
I was also majorly disappointed at Take 2 Interactive's choice of making GTASA a specific hardware game for PS2 instead of taking a chance on XBox and raising the watermark by having a GTASA with true hardware transform and lighting, pixel and vertex shaders and the rest of the Direct X 8.0 effects that the XBox's Nv2X can do.
XBox Tech Specs:
http://www.anandtech.com/showdoc.html?i=1561&p=1
The above link also provides a XBox versus PS2 and GC technical comparison with very technical breakdowns.
On that note, Breakdown was also a let down, definetly. Here you have a Japanese company making a XBox hardware specific game like Breakdown and totally never even considering to extend the replay value with a system link mode, let alone a XBL mode.
I could go on and on, but I am sleepy, I'll come back tomorrow.