The original 5D is still an impressive camera. Aside from the increased megapixels, the mark II has improved high-ISO capabilities, 14-bit Raw rather than 12-bit Raw (allowing Highlight Tone Priority), Auto-ISO, some water resistance, sensor dust control, slightly faster frame rate on continuous shooting, AF micro-adjustment per lens, bigger and better LCD, Live View, and HD movie recording. There are other improvements, but I can't think of any other
big differences.
Here's DPReview's take on the differences:
Overview:
http://www.dpreview.com/reviews/canoneos5dmarkii/
Details 1:
http://www.dpreview.com/reviews/cano...rkii/page3.asp
Details 2:
http://www.dpreview.com/reviews/Cano...rkII/page4.asp