DVD technology is a mature technology these days. Therefore, the differences are going to be mostly in the feature set and the decoders. for $400, any player you get will perform oustandingly. I think that the DVD player you choose should depend on your needs.
For example, do you have an HDTV with DVI? If so, consider an upconverting DVD player. That is, unless you find the decoders in the TV are top notch and you want to use them. In that case, any old 480i player will do - your TV's superior electronics will de-interlace your 480i image, convert it to 720p (or whatever) and display it - that is all the upconverting players do - they convert the signal to match that of your display (hopefully). Both sets of electronics are prone to error. But both sets are going to do the same thing. It is just a matter of when, in the chain, you want to convert the signal.
Therefore, unless you're really into SACD or burned DVDs (which are still fairly rare - and which should still play on most newer DVD players), I wouldn't blow a ton on a player.
Quality must be taken into account - but assuming decent connections which aren't causing audio pops and fuzzy video, consider going with a recognized name-brand in a 1 or 5 disc carousel and be done with it.
