The obvious improvement, (which of course does not mean that it will be implemented) will be to provide full unicode support. This would be fantatsic. OS X seems to lack a full featured unicode word processor.
Ligature support would be nice too (and something the windows version doesnt have, so i doubt we'd see it)
The XL VB studio could do with some spit and polish. some of the windows keyboard shortcuts do not exist (or at least i havent found them yet. Crtl-SpaceBar being the one that i miss most)
All this of course assumes office for the mac will be updated.