Thank you everyone for your responses!
I overall agree with the advice given, and while I have been entertaining the idea of jumping to Nikon, I think the advice that I should wait until I have more experience under my belt is strong.
I went with Pentax because I was enticed by well-priced primes - I like prime lenses better than zooms because you tend to get more IQ bang for the buck, faster apertures, and size reduction at the cost of some flexibility.
At first I found all sorts of deals in the prime category - in the manual focus world. Not a bad place to be, I am learning, because if you actually get good at manual focus it can be much faster than AF.
But when I went to look for AF lenses in the used market... well I can't really find any. The few I can find aren't really deals. Now maybe I was under the wrong impression - by value I should have understood that to mean high quality for the money (which has been my experience) instead of cheaper than brand X. When I began doing comparisons, I was beginning to believe that I had to pay a premium for Pentax, but maybe the truth is that Pentax does not offer much in the lower quality department - there is no FA50 1.8 anymore, for example.
Overall I really appreciate your criticisms of my thought processes. When it boils down to it, I think the issue is that Pentax only offers good or great quality glass, and does not offer much in the cheap category. Of course, with backwards compatibility, I think that there are a plethora of good deals in the legacy department!
I had a great chat with my pro friend this weekend (he's a Canon fiend, but he's very balanced when it comes to brands). He told me that I should not be so reliant on the metering inside the camera, and that changing brands would not really help that. Instead, he told me that you just need to learn the light so you don't need a computer to tell you what to do. I think that was very sound advice.
I think I need to buy a good light-meter and not rely on the in-camera metering, and continue to use MF so that I can become better than any AF system
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As an aside, I do have one question - the K7 and D90 are about the same price, and both have an autofocus motor inside of them. So what are the disadvantaes of the Nikon backwards compatibility that some of you hinted at? I am curious because for the same price of a Pentax M lens, one can get a Nikon A (it's not called that, but you know what I mean) 50mm 1.8. The metering would presumably be better with the A lens, no?