Interesting discussion that I thought I would contribute to.
First a few caveats. I am playing my way up through the difficulty levels and have only reached warlord so far. For this reason there my be AI strategies that I have not yet experienced. I usually always play on huge maps with epic speed, which means certain things for the happiness levels, likelihood of runaway AIs etc.
Secondly I would like to state up front, that I don't think Civ V is the worst game ever. I think it has flaws. Flaws, that for a large part can be expected from a game that changes so many mechanics in a franchise and flaws, that (in my opinion) can and will be corrected in patches and expansions. Let's be honest. Civ IV is so far the greatest incarnation of Civilization, but it only reached its full potential after two expansions and a great deal of balance and AI patching.
Let's start with the good.
1) 1UPT. I think this is an inspired decision. It introduces a level of complexity to the tactical aspect of combat that makes it fun even for me, who has almost never pursued the conquest victories. Granted, there are problems with the AI at the moment making combat easier than it should be, but this can be corrected.
2) Religion is gone. Don't get me wrong. I loved the concept of religion and I would have liked to see it implemented in some form in Civ V, but in Civ IV it had a much too large impact on diplomacy. If you ended up next to Isabella, you had to hope and pray you didn't both found religions.. Frequently half or more of the diplomacy modifier points were because of religion. While this may be realistic, it makes for a poor game mechanic and less fun.
3) Hexagons. It might seem a minor thing, but for me this was an important and much welcome change. More realistic, more inducive to strategic thinking, and less exploitable.
4) Less game-changing Wonders. I realize people may disagree with me here (and I have read several people voicing just that), but for me I like that Wonders give fewer game winning advantages and are more situational. It does tend to remove some of the awesomeness of building one, but I'm prepared to give that up for more fun.
5) Late game is much more fun. In previous Civs (even to some degree in fully patched Civ IV with all expansions) the end game was pretty much a question of inertia. If you were ahead you were going to win
On to the bad:
1) Diplomacy AI and AI options in general. Did anyone else see the mock preview of an update to diplomacy, that had all the options replaced with "Declare War"? This is what diplomacy feels like to me. I think for a large part this is due to me not knowing WHY the AI is mad at me. Did someone else pay him off? Did I move to close to his cities? Secondly there are problems with some of the mechanics here. Yes, 150 turns ago I promised I wasn't going to attack you, but three ages and multiple backstabbings from your side later, I don't feel I have much of a choice. Why does everyone else suddenly hate me for that? I also seriously lack any means of making friends with the AI. Giving presents or having positive relations for long periods of time seem to have no effect whatsoever. At least none that I can see, which leads me back to the point about the "blindness" of the diplomacy system. I feel that much can be improved here by adopting Civ IV's point system (which I feel quite confident they have running under the hood anyway. Just show it to us). Map trading and tech trading are also sorely lacking.
2) Cavalry. I haven't had all that much experience with it yet as I tend to follow the peaceful roads to victory, but my game as Alexander made it abundantly clear that swordsmen and longswordmen were the most superfluous units imaginable. Playing as Arabia or Mongols I doubt you would need archers either (which are really only there to deal with spear- and pikemen anyway). I have read many others complain about other aspects of combat balancing, but I haven't had the experience to confidently claim anything else.
3) Buildings. I realize that the approach is fundamentally different than it was in Civ IV, but I still feel there are problems in this area. Can anyone tell me what the point is of having a harbor, except in the one or two cities you produce most of your navy in? Maybe if you play Archipelago maps, but shouldn't the building be useful in all games? Solar plant are great for those desert cities placed primarily for oil ressources or the likes, but why does it have such a huge production cost when the cities that can build them typically have less production than most cities? The usefulness of Forges has been questioned by people more skilled than me (referenced by Masterminded above).
4) Terrain. This is probably one of my biggest grudges. The sameness of terrain bonusses alongside the meager bonusses from special ressources are ruining a lot of the fun of the game for me. This may not be true for everyone, but I'm one of the players who spent a lot of time planning my city placement and making sure that every ressource was optimally used. It was one of the primary reasons for me declaring war. Catherine settled her city so that Barcelona lost a cow ressource and the supercity I was planning is no longer possible? She must die! Now every city but the ones settled on ice or desert has about the same output, regardless of special ressources. Where's the fun in that? One exception of this sameness is the crippling sparsity of production. Basically you are going to want every city to be surrounded by forest. This is by far the most useful terrain and if you need food or gold you can just remove some of them for a handy production boost as well. A city without hills or forest is nigh on useless. Bringing back the Civ IV workshop would correct much, but an overall revision is much needed.
5) Diplomatic Victory. I would like this to include some element of skill rather than paying off or liberating enough City States to secure a majority.
6) Random events. I know a lot of people didn't like them, but could we please have them back as an option. I felt they added to my immersion and roleplaying and were just plain fun.
7) ICS. I saw hints of this superstrategy in my first games and it has later been proven to be a staggeringly powerful strategy on all difficulty levels. What I dislike most about this is that it goes directly against one of the stated purposes of Civ V to make smaller empires a good idea. Something needs to be done soon to correct this.
8) Runaway AI. In most of the games I've played (Frequently on Continents map type) one of the opposing players will start systematically eradicating everyone around him with seemingly little opposition and quickly take over the entire continent. While this may be realistic and certainly presents more of a challenge for me, it does tend to remove a lot of complexity from the end game and once again go against the "small empires are not necessarily worse"-concept. I'm expecting this to be corrected with AI patching at some point.
9) Advisors. I like that they're back, but some minor fine tuning is necessary. I don't have to have my military advisor tell me that every single one of my cities has military buildings and should be producing units. I know that. I built the heroic epic for a reason and state bankruptcy due to an enormous army was not it. Likewise I don't need the trade adviser repeating every single request from City States. Maybe put those on the diplomacy screen next to what ressources they can give me as allies and use the trade advisor something actually useful. The advisors are a minor point for me but still an irritant.
10) Civilizations. I can't quite seem to put my finger on why, but most of the Civs seem fairly bland to me. Few of the bonusses are really useful to me, the unique buildings are often lackluster or highly situational, and not being a warmonger I can't really get excited about the enormous amount of unique units. I can't really seem to pinpoint exactly where it goes wrong, but this really irks me.
11) Steam. Yes, I know.. It doesn't really belong here and everything (and more) has already been said about it, but it stills annoys me immeasurably. The only winners are the people who downloaded a pirated version. This is not an invitation to restart that particular discussion but merely a bitter old fart reiterating his pet peeves.
Things I am undecided about or have no opinion on:
1) The changes to happiness, research and economy. I realize this is a large change and people more experience with Excel spreadsheets than myself has made compelling arguments both for and against it. For my sake I think it is too early to pass judgement. I see both positive and negative elements and I look forward to seeing how the developers may tweak the systems in the future.
2) Cultural policies. I liked Civics a lot and I like policies a lot. There may be questions of balancing, but this was also true of civics. There may be policies I would never dream of taking, but this is also true of policies. Ultimately the change is not that significant to me.
Generally I just can't seem to get excited about Civ V. I'm hoping for a few other patches or some new civs to spark my interest. It's difficult for me to articulate my disappointment exactly, but I'll give a concrete example: For me the best part of a Civ game has always been the first couple of hours. That exciting period, when you are exploring, adapting to your surroundings, planning your future development and laying the groundwork for your inevitable victory. Yet in Civ V I find myself getting bored in this part of the game. It's not that there aren't things to do. I just can't seem to drum up much enthusiasm for them. The end game is much better sure, but I still feel sad to lose that first rush of excitement.
I haven't given up on Civ V. I'm not going to make some hysterical statement about how I will never play a Firaxis game again and how Shafer has ruined the franchise. Frankly, I find that kind of behaviour insufferably childish and not a little ignorant. I recognize the excellent foundation that Civ V represents and I hope that Firaxis in cooperation with the community can make it a great game in its own right.