Near as I can tell, the only difference is that they added this concept of tracked vs. untracked files. When you do a commit, everything that's currently tracked gets included in the commit unless you specifically list a set of files to include or ignore when you make the commit. As a side effect they seem to have removed the concept of a staging area.

Functionally it's a lot like every commit has the -a flag set and anything that's "untracked" is in your .gitignore file. I've not seen any other differences, although some other comments have mentioned some difference with regards to the way branches are handled.