I can only compare my direct experience (PhD Social Psych) with my former co-worker (Med School) and my wife (Law School).
Med School is a lot of required courses, and tough ones at that. The MCAT as a test actually tests knowledge, which means you have to know more than 9th grade math and have a good vocabulary (like the GRE). From what I've been told, most med schools (not the very top or the very bottom) are comparatively good. You may have a preference, but when you get out, you're well-trained and should be able to find work no matter where you go.
Law School is a very different beast. Your income will more or less be dependent on the rank/tier of your school and probably the rank in your class. Bottom of your Yale/Stanford/Harvard class will still get you into a top firm at $160k a year, if that's what you want. Your rank becomes more of an issue when you get to programs ranked 4-14, then again from 15-50. Once you hit the second tier, you start to have certain doors closed to you (probably no Cravath, no Wachtell). And getting into these programs is mostly dependent on your LSAT score and your college grades. Law schools accept lots of people, though. I believe Harvard accepted something like 700 people to yield around 500 in 2005.
For psych, well, fit is key. I can't speak to clinical, but I don't think there were far more than 700 acceptances across all social psych phd programs in 2005 (unless my math is way off), and definitely yielded less than 500 (with multiple acceptances chipping away at the yield). Then again, most of those programs were fully funded... med schools and law schools typically don't give you a free ride.
It makes sense, though. If you're getting a PhD, the idea is you'll be contributing something to your field, advancing the state of knowledge. You don't have the same expectation as a lawyer, doctor, or vet. Your duty is to your clients/patients. For PhDs, your duty is also to your profession. At least, how I see it.
I'll be honest with you... I had a good profile, and I got into one social psych PhD program while applying to 8. I easily could have gotten into zero this round. Even with two years of prep, I probably couldn't have gotten into any med schools (and forget vet schools) since I lacked the prereqs and I really don't enjoy bio, chem, anatomy, or needles. Given a year of prep, I probably could have gotten into a T-14 program for law. My numbers were there for grades, and I could have broken 170 on the LSAT. I wouldn't have gotten into Yale, but I would have had an inside track on Harvard (assuming my wife counts as a legacy for me).
But that would have meant spending my life as a lawyer (or I-banker), instead of a researcher. And dammit, I want to be a researcher!