Well strictly speaking there isn't much other than graphics that they'd have to cut to make the game run on the Wii. It's got nifty physics and some huge enemies from what I've seen, but nothing a console with more power than the original Xbox shouldn't be able to handle.
Then again the graphics are pretty much the only reason people are interested in this game, and there isn't much of the hype left if you take that out.
Of course it's entirely possible that EA has realised how poor ports of poor games sell poorly on the Wii, and decided they wanted to port one of the higher profile games. The name of Crysis is so big that it would probably sell well, and this is EA we're talking about here. They're probably planning on starting Xbox 360 and PS3 portion projects right after the PC version is ready, no matter what the original team says or does about it.
