I have found this web site to be a good guide to things like the quality of the transfer/soundtrack.
Blu-ray.com - Blu-ray Movies, Players, Recorders, DVDs, Media and Software
It can vary considerably and there's no point in replacing a DVD if the Blu Ray isn't much better. They also usually will tell you what if any special features there are and how their number/quality compares to those on the DVD editions.
In general, the more recent the disk release is, the better it will be. Older (especially first generation) releases often had shoddy video (well shoddy for blu ray) quality.
Keep in mind that you need a GOOD quality TV (at least 50" and 60+ is preferable) and a good quality sound system to get the most out of a blu ray movie. If you have neither it really isn't worth going with blu ray.
BTW, you don't have to have an AVR that supports the "lossless" audio codecs (TruHD, DTS Master Audio) to get benefit from the improved sound quality. The regular Dolby Digital and DTS tracks are much higher bit rates than the DVD equivalent and thus sound better.
In general the PS3 ($300) is still one of the best blu ray players (plus you get a dammed good game machine) and about as "future proof. Expect to pay at least $150-200 for a good player. It should be "Profile 2.0".
Product reviews and prices, software downloads, and tech news - CNET has reviews on a wide variety of blu ray players. So they're a good source of information. The current "king" of the blu ray players is the Oppo BD-83. It is very fast and has pretty much every feature you could ask for. But at $500 it is fairly pricey. The biggest complaint about cheap blu ray players is that they can be quite slow in operations (starting up, loading disks, etc...). Faster operation is one of the major things more money buys.
I have NEVER gotten that "needs an update" message from a blu ray disk with my PS3. Of course that get's updated fairly often (usually adding new features in the process) anyway.
