What makes a good transport a good transport?

Howdy! I'm curious as to what makes for a good CD transport, either in a dedicated transport or a CD player (w/ internal dac) using its digital outputs. What features make one player better then the other that one should look for, such as component type and quality, mechanisms, power supply or other specifics? More to the point, I'm looking for a used CD transport for $400-$500, or less, on the used market and am wondering which path to take. I'm hoping you good folks would share your experience using various transports available in this price range used. Thanks in advance for any advise.
I might suggest "a good imagination"....

Seriously, in that price range I'd look for a used Sony DVD player, preferably the DVP-S7700. You should be able to get it for under your target range and it's reviewed as an excellent transport. The fact that it's an excellent DVD player too is a bonus.
I use both a Parasound CBD-2000 and a Sony SCD-555es as transports and they are both good, much better than some other highly regarded transports. Both are no longer current, but you may be still be able to find a new Sony for around $600.
Platter stability, laser sled stability, signal re-clocking to reduce jitter, robust power supply.

Personally, I use a Muse Model Five, a superb transport IMHO! Available used for around $500.
You might find an Audio Alchemy DDS Pro for around $500. Kalman Rubinson of Stereophile once had one in a system of his. I've compared it with Sonic Frontiers SFT-1, a tie as far as I could tell, and with an Accuphase DP-75's transport and with a CEC TL 1X. The CEC people believe a belt drive is key, and I can't differ with that. But in your price range, I don't personally know of any that beats the DDS Pro. (I haven't experienced the Muse, which should be good too.) Provide good isolation from vibes, though. Not doing that could wipe out big quality differences.