Wheels - your frame basicly just holds them apart, the steering points them in a given direction, and the drivetrain makes them go round. But wheels are the heart of any bicycle, and it continues to be a mystery to me why its so typical for manufacturers to put cheap wheels on even expensive bicycles.
Personally I'd rather have some high end wheels on a reasonable frame -IMO the average rider taxes the limits on their wheelset far more than their frameset.
I've picked up some Halo Spin Doctor hubs and plan on doing some axle mods before lacing them up. These have cartridge bearings and should be possible to replace with a ceramic hybrid.