a full frame sensor and a different body construction. but you won't see a need for those. The t2i is more than sufficient, plenty of resolution, more fps than the 5d mki, shoots video, and has a crop sensor. plus the t2i or some other camera is cheaper. you can buy the best canon they offer right now, it'll be replaced by next year.
motherdrizzlinSL: no, it was disturbing. I thought my fecal matter was going to retreat back into my bowels.
get a t2i or something similar from canon, or a D5000 or something similar from Nikon, I don't think you understand the circumstances in which someone "grows out of" their DSLR, you should be more than fine for a while while you learn the basics