It seems all audio related topics have been pretty well discussed, although I'm not sure the number of channels is all that hard. My experience has been that once you get everything hooked up and room correction done, that part of the AVR experience mostly works pretty well.
I say mostly, because I'd like to suggest the pain points in the AVR world, for the average non-technical person, lie mostly in the following areas:
1. Monumental, yet still obtuse, users manuals. I've had several Denons (and Sony and Yamaha) and the User's manuals, while typically complete as far as they go, are woefully incomplete in terms of describing why and when a particular feature is used. Examples: What to set the LFE level and crossover frequency to, when to use LFE vs LFE+Main, etc., etc., There is a whole body of knowledge one is expected to know at the forest level, the manual only deals with the trees.
2. The unpredictable weirdness that is HDMI, especially when things like ARC and EARC come into play. (why can't I see the on screen displays anymore when watching TV, etc., etc.), cables of varying qualities and speeds, it goes on (copy protection, aargh.)
3. The whole issue of CEC, if one should choose to use this admittedly seemingly pretty useful feature, but you'll go nuts trying to predict what will actually happen in many cases.
4. Just the sheer magnitude of formats and codecs, etc. both in the video and audio realm (just in the Dolby world alone, never mind the rarely used DTS and Auro modes).
These things are one heck of a box of tricks, probably the most complex devices in the modern home today (let's leave computers out of this).
It sure seems this whole user interface and reliability (predictabilty) area is primed for disruption, but sure as heck not by me. I think it's one of the reasons Bose and Sonos do a pretty good business.