Best answer as chosen by user carlapappalardo: Canon's APS-C Sensor
Canon hasn't brought out a new sensor in seemingly forever, meaning that the T4i has the same sensor as the older T3i, and the T2i before it. It is also the same sensor used in the 60D and 7D. So while the "prosumer" models have more "pro" features, they also have very similar image quality. As far as the 60D goes, as you note it has some weather sealing, along with a more robust battery, a brighter pentaprism viewfinder, a top deck status LCD, two control dials, and more buttons for direct access to commonly used controls. The 7D adds a magnesium chassis, a more sophisticated auto-focus system, and faster and more robust performance.
Full frame does net better image quality over a wider range of conditions, but at a significant cost in size and dollars. Naturally he manufacturers are keen to sell you one because they make more money not just on the camera, but the lenses as well.