It's hard to pin down exactly what the root cause is as everything depends on everything else. My view is that the single one thing wrong with Civ 5 is City States! I agree that the basic mechanics of the previous 4 versions has been more radically changed than ever before, but I think this has less to do with hexes and 1UT than City states. City growth depends on getting the over-powered and unbalanced freebies from the city states. I tried playing with no city states as interracting with them irritates me - it seems like a "whack-a-mole" mini game. But the game doesn't work without them as it seems the cheap food from the CS has been countered by low food production tiles.
Hexes are OK, but they really don't add any *substantial* strategy. I think they're more of a gimmick than adding real gameplay value. 1UT is good, but I would like to have achieved it by allowing merging of units into 1 counter/unit. e.g. merge 3 swordsmen into 1 swordsman counter with 3 * the abilities.
Also, a whole branch of Social Policies is for city states. Some buildings and wonders relate to them...much as there are aspects of 5 that I prefer to 4, switching off CS means you may as well go back to 4.
What baffles me is that as there is now LESS micromanagement in Civ 5 (which is generally a good thing), the AI should be more effective and faster. Think about it: far less units, squares have 8 exits, hexes 6 so pathfinding should be quicker, less cities, less details all round than Civ 4, but still the AI turn is dramatically slower. How come?
Overall, Civ 5 seems like a repeat of Civ 3. Made some risky, radical changes from a previously brilliant version. Some good, some not so good, with the changes causing serious unbalance in places. Perhaps like Civ 4, Civ 6 will be the one that refines to a true masterpiece?