Is it too late to ask for a more simplified Twentytwelve as a default theme ? I think that Twentytwelve's CSS is too complicated as a default theme.
The CSS part of the theme is way bloated with px fall back just to support text-sizing of old IE which is not a problem because IE already has buildin Zoom (Ctrl+/-/0) The size of CSS is way too big and now it has separated CSS file for old IE too !
I'm not saying that we should giveup IE support, but the px fall back are way too much, think about the performance aspect, why load all this fall back while its benefit is only for very little usecase.
It's better to just use rem without px fallback.