The ACD supports 720p. Like most 20" flat panels it has a 1680x1050 resolution... more then enough for 720p, not quite enough for 1080p.
I'm not sure what you mean by catch... you don't really have to give anything up (other then more cash) to get a display with more then 720 vertical pixels.
Compared to other 20" displays (like the dell 2007wfp) the ACD is significantly more expensive, has fewer inputs (only DVI), and has a worse contrast ratio / response time. It is however more color accurate, has SWAP certification, and looks much better then the Dell.