Technically speaking, you want lower THD (0.5% is better than 5%) & crosstalk (-70dB is better than -20dB), higher SNR (80dB is better than 40dB), flatter frequency response (20Hz-20kHz +/-0.5dB is better than +/-5dB), and good power output (100 watts @ 8 ohms/ 150 watts into 4 ohms is better than 50 watts @ 8 ohm/ SHUT OFF @ 4 ohms).
Probably the most important is the true power handling. If your speakers has a minium impedance of 3 ohms, you probably should get an amp that is rated for 4 ohms or lower.
As far as THD, crosstalk, SNR, & Freq. Resp., even a $300 Onkyo can have better numbers/specs than some $3,000 pre-pros! So we can usually throw these numbers out the window.
The 2 areas that separates a $300 receiver from a $3,000 receiver include 1) features 2) build quality.
Most people don't plan on keeping their receivers for more than 5 years because we have "upgraditis"

, so the build quality is not so important. So the primary difference is the features. Do you need a Silicon Optix Realta video chip? Do you need 6 HDMI inputs?
As mentioned already, when you compare 2 receivers or pre-pros or preamps/amps, as long as everything else is equal (source, playback modes, volume, etc.), you will most likely get very similar sounds.
Now in my experience, some preamps/amps may have "slightly" warmer or brighter sounds, and I emphasize the word "slightly". It's rather subjective, but some of us think that we can hear it.
Oh, I forgot. Some of us are just plain crazy and buy things because of factors like aesthetics and so-called "pride of ownership". It's pure personal preferences and madness.