DEN has a couple of shortcomings, neither of which have to do with the terminal design.
1. Retrieving checked bags can be hideous. It often comes a close second to UA's worst, SEA. Though DEN has gotten a little better in the checked baggage department in the times I am absolutely forced to check a bag.
2. I despise coming in over the Rockies in the summer after 3:00pm. Does not make for a smooth ride.
Quite possibly, some people's disdain for DEN might be the long, straight walk down B. It can seem like you're not getting anywhere, but you've been walking for a long time. At least at ORD, you inevitably arrive at B1 and depart C32....you get some variety en route such as an escalator, neon tunnel and an oversized dinosaur (not to be mistaken for a TSA employee).
I agree that IAD is the place from hell. I avoid it at all costs.