main differences are:
build quality (magnesium alloy body vs plastic)
FPS (8 vs 3.7)
Processors (dual digic 4 in 7d vs single digic 4 in t2i)
Autofocus (19 pt vs 9 pt)
and customization/menu features.
But really, with the magic lantern firmware, the t2i gains kelvin WB, intermediate ISO's, and more control over video than the 7d. If using for photos, the 7d is somewhat better, although it has pretty poor noise handling at lower isos.