<shrug> who knows - its probably a mix of how much research is going on, so that you know the faculty are very up to date on whats new and upcoming - a good mix of patients - that offers good exposure, excellent teaching faculty - they are trained well and know medicine as well as psych, and good residents so you can learn from eachother. Lots of fellowships or fellowship opportunity, academic track, elective 4th year, can run iwthout the residents so you're not a work monkey, scholarly activity always around - i guess those things would make a top tier right?
I suppose by name alone those "top tiers" are supposed to have all that, plus the name to boot. But I think you can definitely find this stuff at smaller programs and its like someone else said in another thread: do you wanna be a big fsh in a little pond or a little fish in a big pond? I'd rather be big fish.