My BIG (MASSIVE!) complaint with Canon full-frame DSLRs, is that they never include an onboard flash. Whereas such was commonplace on film-based full-frame SLRs.

I WANT a flash on my camera body. It is incredibly useful for fill-light purposes. Indoors, I would probably never use it, preferring slower exposures or a much more capable "external" flash unit, or even my elaborate "lighting kit." But outdoors, the strobe on my 40D gets a lot of use.

That's the one, the only, thing blocking me from updating my antique 40D to a full-frame DSLR.
Oh, that and the collection of cropped-frame lenses I seem to have accumulated in the interim.