Are all preamps created equal?

What are the important things one should look at when looking for a mic preamp? In particular, what exactly drives the price differences between the "more equal" and "less equal" brands? Quality is obviously the thing, but it has many measures (noise, harmonic distortion, headroom, etc). Say how much more are you getting say from a UA class A preamp vs. say a standard, good-quality mixing console pre from a good brand like Yamaha or Tascam?