Is it aimed at any particular domain? Finance folks pretty much live in excel, they might make a good use-case on what can't be done or what they don't like in excel.
While I am not a big user of excel, the google docs version of excel is very similar to the desktop version + collaboration (think Oddpost). The other approach taken by some people I have spoken with is viewing it more as data and tying to do more (think gmail to some extent).
Slides 4 & 5 give some insight: http://www.scribd.com/doc/2191289/yaron-minskycufp-2006
While I am not a big user of excel, the google docs version of excel is very similar to the desktop version + collaboration (think Oddpost). The other approach taken by some people I have spoken with is viewing it more as data and tying to do more (think gmail to some extent).