I really love MPC and use it for most of the stuff I watch. But sometimes I switch to MS Media Center 2005 for one reason, the intelligent zoom.
4:3 video looks awkward when stretched to 16:9/16:10. Media Center has a feature called intelligent zoom which stretches the image, but somehow doesn't totally screw the proportions.
This site has sample pictures
I would be really great if MPC could include a similar feature. This way I can watch 4:3 content full screen on my 22" monitor.