I always liked - and still do like - the "old" mail app icons, just because they have colour. The "new" icons look plain, simple (guess who's idea that was - SJ) and surprisingly out-of-date; the "old" icons look much more slick, with the nice colour icons and the flat - shiny, not old - dull look.
They are supposed to be bringing thing "Back to the Mac", the springboard on iOS has colourful icons - but then again the mail app has basically no colour to it.
Mac OS X 10.7 lion's colour scheme is turning out to be pretty shocking. (bad)