What about the menial bug where Media Center keeps sending you to Recorded TV when you select Program Guide?
That is an annoying bug. It seems that for some reason, Media Center doesn't change something "under the hood" when a left or right key press is received, but it magically starts working if you use up or down. Why? Because the magical fix for access the Program Guide is to hit up then down (or down then up) and then hit right to access the Program Guide.
EDIT:
There's also another issue where Windows Media Center tells me that my setup isn't HDCP-compliant. This seems to only occur after a reboot, and it appears to go away if you windowize your program (i.e. hit ALT+ENTER). It's still rather annoying since the nVidia Control Panel reports my setup as being HDCP-compliant the whole time.
Although, I need to look up a way to have WMC launch XBMC for me, because while I prefer WMC for its tuner capability, I still prefer XBMC for playing networked media. I tried out Media Browser, but it just doesn't seem that worthwhile for me. One of the largest issues is that it doesn't seem to have an anime scraper, which means when you browse my anime, you are presented with a bunch of question marks.

Also, I prefer being able to see more at once, which means I like XBMC's File Navigation Mode over the fancy movie poster mode that Media Browser has (XBMC has a similar feature). I can also sync my libraries across my XBMC instances using MySQL, which means I can watch an hour of a movie downstairs, go upstairs, and continue watching it where I left off with no effort (other than hitting enter on the "Continue from..." menu).