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.