Greasemonkey-a-like geared towards close integration with browser chrome; should eventually be possible to re-implement many major extensions using it.
Google acknowledges that some people are going to change their own experience of our web applications regardless of what we do. Resistance, as they say, is futile.
Takes existing Greasemonkey scripts, but some (a lot?) won't work -- I imagine that lack of XPath support will be a huge pain.