I'd also really love to see the percent grade shown. This is about the only thing I use Garmin Training Center for.
Specially, in the Player it'd be very useful to be able to plot Power/Heart Rate/Cadence/Speed vs Percent Grade. As a climber, that's the most interesting thing for me while training, and it's too hard to pick out steep sections of a climb just from an elevation profile.
Displaying a "Max Grade" for an activity would also be interesting for bragging rights.