Thanks for the suggestions, they're all good ones!
Inset shadows have been at the top of my list since day 1. As you read in that other thread there are severe limitations with it, however, and despite several tries at a real implementation I've always failed. I might try to do a limited implementation that doesn't work with border-radius, but I'm not committed to it yet.
I do want to flesh out the preliminary work in the text-shadow branch, so it's possible that will make it in 2.0.
Radial gradients are probably never going to be possible in IE 6-8, but I'd like to try getting it done in IE9 for 2.0.
I'll take box-sizing into consideration.
The margin-top workaround sounds like it should be simple, so I'll bump that to the top of my list. I'm leaning toward making it something you have to explicitly enable with a custom CSS property, to prevent unwanted side-effects, but I'll give it a shot without first.