Game's getting one hell of a beating at the Civ Fanatics' Forums, and I have to agree with most of the views here. Here's what they think in a nutshell:
- Hex Grid - infinitely better than square tiles
- 1 Unit per Tile - no stacks of doom, battlefield tactics and less worrying
- City Defence - no more babysitting cities!
- Improved Gold dynamic, allows gold to be actually used instead of just spent on research/culture
- Resource dynamic - only as many units as you have the resources for
- City-States! Limited wars, goodies, quests, and lots of flavour.
- Better balance of options; warfare is easier to manage, cultural victory is now feasible
- Social Policies are nice, they can get addictive if you play for the cultural victory. Unfortunately, their permanence means you cannot have any dramatic revolutions. No French Revolution or Nazi Germany or dramatic shift from Monarchy to Democracy without wasting the culture you spent on a whole tree. Civics was better in that respect.
- Stupid AI. Absolutely doesn't know military tactics, makes senseless diplomatic decisions, and is either a.) puny civ to be conquered soon, b.) juggernaut taking over the world.
- Blind Diplomacy. This one is more a get-used-to-it thing, but diplomacy is now less predictable and involves keeping a mental track of your relations with other civs. Otherwise, all the civs end up not trusting you and brushing you off.
- No flavour. The game seems to be entirely concentrated on being gamey, and not so much on the fictional world within it. The leaders never have anything interesting to say, they don't seem to have proper personalities as leaders, but appear to be players who are simply in it for the game. Ultimately, it makes the game feel boring because you're playing just to play and not really for the whole civ-building experience.
- No graphs or replays! Maybe they're geeky, but it's great to see how your civ progressed from a sapling into a world-conqueror, or when you adopted what policy. Now, if it's over, you're just given a pat on the back and told to move on.
- Interface. Big and clunky. I find it usable enough, but it really doesn't have to be so huge and obtrusive. Civ IV's interface was more elegant than this. The artwork is gorgeous, though.
- Performance/Stability. Maybe I don't go well beyond the minimum graphics, but the game looks barely as good as Civ IV with lowest graphics and yet seems to run less than half as fast. Ultimately, I'm playing in strategic view - easier and simpler. Stability is an issue, as Rake reported - it seems that tons of people are getting CPU/GPU overheats with the game and I've had 4 game freezes so far.
- Turns taking forever. Civ V is by no means the first game suffering from this, but there's no reason not to complain about the glacially boring late game, where you just click End Turn, wait 20 seconds, click End Turn, rinse and repeat till that spaceship component gets built.
- No Map Trading? Probably to balance the Natural Wonders thing, but it makes little sense that you can't trade world maps in the Industrial Age, or see the whole world if you've researched Satellites.