The biggest challenge is separating the useful classes from the useless classes. If you check our your CSS, my guess is that there will be a lot of style information, and if we were to put all of into a drop down for you, it would be difficult to use (since the useful styles would be mixed with the useless styles in one long list).
Great suggestion, though, definitely something that we'd like to do!
Charles Teague (MSFT)
Windows Live Writer