main thing i get from YSlow is to use Expires tags. all threads get an 'F' grade for that.
yahoo also recommends distributing your includes over multiple locations, which ain't gonna happen -- so they ding you for it in their overall grading system.
they then pop you for not compressing .js and .css files.
so it's perty hard to get a grade of 'A' from them.
it's still interesting info, though and they have some good recommendations on where to be including .js and .css in proper order.
btw, this site got a grade of 'D (65)'.. which indicates to me that it isn't a grade that indicates true site performance.. but merely how close to what yahoo would want