The fast/slow fade's we've already got are limited and don't look too great. I want to have more control over my effects, transitions and media generators by being able to use some kind of graph editor to smoothen things out.
Actually you could apply velocity envelopes onto the build-in transitions (except of to a simple dissolve) which gives a rather good control over transitions. And for FX there are bezier curves to offer more manual control (though there seems to be a bug in the FX bezier curves).