Unless you are going to run more than 7 speakers I think you'd be fine with the 33XX series. I wanted 9 or 11 speakers so I didn't have much choice. The 4520 has a nice feature set and gives you the option to use 9 or 11 channels but as far as superior sound quality, I don't hear it. I know they claim various sonic improvements and they're probably in the box but my ears can't hear them. I like Neo:X a lot and I'm sure the upgraded Audyssey helps (especially with dual subs) but if I didn't run 9 speakers I would have been plenty happy with my 3311. That being said, I've had no issues and I do like this AVR, I just don't think it's worth the money unless you are going to use more than 7 speakers.
BTW, my speakers are Paradigm which are very efficient and sound fine with this and the 3311. I doubt that power would be an issue unless you're using the most inefficient speakers out there.
Also, and I've said this before, but what is it with the Denon manuals? I'm not the smartest guy around but I don't really consider myself stupid............that is until I try to figure something out in the Denon manual. I can read 5 pages of stuff about any given subject in that damn book and know very little more than I did before I read those 5 pages. It's like they give you a ton of info but without really explaining anything. I still rely on Batpig's Denon for Dummies.
I don't see how anybody (unless you get a bad unit) could dislike this receiver, but IMO it's not worth the additional cost unless you're doing 9 or more speakers.
Also, my experience is the exact opposite of Cvetan1"s regarding heat. At least in my environment, the 4520 is considerably hotter than the 3311 was.