With the exception of Select Certification and Surround EX, those are all features that let you tune the sound. My receiver (Ultra certified) has them but you can get them (without the THX name correlation...) in many receivers. Whether or not you need them is dependent upon how close to ideal your room is and how precise your ears are. I wouldn't be surprised to find these features lacking in low end models from Sony, JVC, Pioneer, etc, but I would expect them in Yamaha, Denon, Marantz, Outlaw, Onkyo, etc.

A quick google search on each would probably give you more detailed info than I can list off here but I'll sumarize where I can.

THX Adaptive Decorrelation - attempts to break out specific sounds from mono signals to fake a surround effect.

THX Bass Peak Level Management - Prevents high bass peaks from shooting into your subwoofer and frying it.

THX Front-Channel Re-Equalization - THX claims this results in "smoother high frequency response". I've never been able to determine what exactly that means but it sounds an aweful lot like lowing the gain on high frequency signals to prevent sharp, high tone noises from screaming through your room. Some might hear that and find it more appealing. I prefer to trust the director to present sounds for the effect they want and I assume most here agree or they wouldn't be buyin typically VERY accurate Axioms.

THX Loudspeaker Position Time Synchronization - This institutes a tiny (probably imperceptible to most people in most rooms) delay in order to ensure all sounds reach your ear simultaneously.

THX Select Certification - supposed measure of quality.

THX Subwoofer Crossover - adjustible sound filter for LFE channel.

THX Surround EX - Some Dolby Digital signals have a rear channel matrixed into the right and left surround channels. If you have a 6.1 receiver, it will pull this out and move it to the rear. Non-THX receivers do this as well, just possibly through Dolby's algorithm instead of THX's. The mixer encodes with Dolby's algorithm in mind.

THX Timbre-Matching - attempts to match the front speakers to the center. Assuming you're buying voice-matched speakers for your HT in the first place, this should be a non-issue.

Some of these features may be important in your placement, some of them may not. Whether you need them is best left to you to decide. I doubt most consumers shopping for a low end receiver (I consider anything sold in BB or CC low end) are pairing it with equipment nice enough and are being careful enough with their placement to actually hear what differences they make. If you want them, look for them.

If you want to know about the various sound formats (DTS, DD, Neo:6, DPLII, etc), start another thread on the subject and I'll answer there as it's a bit outside the scope of this thread.

Regards,
Semi