I had a 5D and replaced it with a 5DII.
Much better rear LCD so I don't have to go (more) blind looking at the histogram...etc.
Micro Adjust. I've needed it for a couple of my lenses.
More pixels to crop with. If you do little cropping or don't print larger than 8.4 x 11, you'll see no difference in the prints.
If you shoot at high ISO, > 1600, the 5DII will give you a bit more than 1 extra stop to play with. At 1600, it's still much better but the 5D was, usually, good enough for me. I shoot a LOT at ISO3200 so that was a very important reason for me going to the 5DII.
Small things like ISO in the viewfinder. I forgot more than once to change my ISO after a nights shooting. No big deal but an improvement, nevertheless.
That's about it. Mainly more ability to crop and better high ISO performance.