The basis of most people's complaints seems to be "It's not Civ 4". Apparently some people never played any of the Civ games before 4 and realized that they change every time. This isn't Madden.
See that's not exactly the problem. Yes, it's not Civ IV, but no one expected an exact clone anyway. We knew we were getting ranged combat, hexes instead of squares and we new a bit about social polices. We knew religion, spys, and transports were gone too.
What most of us disappointed people didn't anticipate was a return to city spamming, ultra aggressive AI that just wants war, war, war. AI that doesn't even know how to use a boat, social policies that get locked in. We didn't expect tile bonuses to be so underwhelming, moving armies to be so tedious and the need to buy land you already own.
Who would have thought a simple ways to remap keys would be missing, or even having so many keys changed from the old games to something you can't even figure out now. Scoring is changes, the lists went from something meaningful to 'the most shiny people in the world'. It all just seems to uninspired, consolized and mechanically broken.
The problem it seems is that Civ IV: BTS was so feature packed, balanced and sufficiently challenging that we thought Civ V would build on that, not strip it all down to basics and start all over.