I think Randy covered it well. HK and Denon (along with Rotel) have always tended to have beefier power supplies and power amplifier stages -- not necessarily more watts, but more "headroom" at the same wattage and more ability to drive that wattage into difficult loads -- rather than spending $$ adding features.
One way to put it is that HK, Denon and Rotel receivers have been felt to be a bit closer to the construction and design you would get with separates. Some people believe this makes a difference in sound quality under certain conditions (the beefier power supplies etc, not the "separate"ness) while others believe that is just placebo effect, and nobody knows for sure.
In the last year or so Pioneer seems to have been making some really nice receivers -- the 1015 was the first to get noticed -- and the latest crop of Yamahas are rumored to behave better with 4 ohm loads, so in some cases it would be fair to say the gap is narrowing.
My personal view is that Pioneer, Yamaha and Sony all offered a couple of different ranges of products, where the top range was pretty good and the bottom range had lots of features but often very lightweight amplifier design, whereas Denon and HK took a different approach, where their low end products had fewer features and less advertised power but maintained the design quality and would often seem to outperform units which appeared much better on paper. As a result, people who were really into audio tended to spend their money on the HK and Denon products, even if they got fewer features they felt they were getting a better designed product.
As Randy said, once you get into the upper midrange receivers the difference in features at a price point pretty much goes away. It's only at the low-midrange price points where you trade off features for "beefiness".
Well, this post should upset just about everyone on the board somehow. I hope it helps answer your question
