Civ 5 is the best goddamned Civ game ever; I've already spent 12 hours playing it, and I can see myself spending hundreds more.  however, there are a LOT of things that are janky and/or outright buggy about it, in grand Firaxis tradition. 
 
janky shit: 
- from the "Change Production" pop-up, you can't see yield icons (unless you turn them on across the map), you can't see which tiles are being worked, and you can't see a list of other buildings in the city.  this makes it really difficult to tell how good a "+1 Production for every Sea Resource being worked" or a "+1 Food for Sea tiles" building will be for you. 
- in the run-up to the game's release (and, in fact, stated explicitly in response to a question at the PAX demo I went to) it was said that spearmen can't destroy tanks any more.  but longswordsmen can still damage helicopters, and I can't imagine spearman/tank encounters will go any differently.  
- helicopters are still a threat.  they also still have the inexplicable Civ 4 features of having unlimited range and being unique among air units in that they're able to conquer cities by their lonesome.  and they've added the new inexplicable feature of having to embark and drop 1/3 of their movement speed to cross straits/fjords/other narrow bodies of water that they can cross in a turn.
- it appears that unlike city-states, if you liberate a civilization, their leader's diplomatic stance toward you doesn't change at all.  the instant I liberated Russia, Catherine resumed calling me "O bloodthirsty one" and saying my cities were "a plague on the earth." 
- I'm not sure whether this is intended, but it's possible to be on such a bad footing with someone that they won't accept gifts from you.  Oda Nobunaga refuses to allow me to give him horses.  
- it seems that, if a tile is not visible or otherwise somehow remote, the game will sometimes assume that a unit can only move one tile per turn.  as a result of this...
- pathing seems to break hardcore if a path would involve embarking.  units will routinely go the wrong way around the world because they're convinced that a big section of the "right" path can only be moved through at one tile a turn. 
- the lack of stacking means that you can't ever order a unit to move to a tile that has a unit on it right now, even if it would get there 10 turns later.  this gets mad annoying if you (say) know exactly where you want a unit to be, but an automated scout or something is in the way. 
- you can't stack civilian units, meaning that if you have a spare great person lying around in one of your cities, you have to do a cute little unit dance to get a spaceship part or some other civilian unit to move to the city. 
- if you build a wonder and get interrupted by someone else building one, your lost production gets converted to gold.  if you start building something and then purchase it (which doesn't cost any less than if you bought it outright at the beginning) or switch away, you just lose that production.
 
bugs: 
- on a machine with the latest Forceware and a GeForce 8800 GTX, the DX10/11 binary is crashy as hell (consistently ~5 minute uptime.  once it lasted 20 minutes.  that was a good day.)  I've fixed this by just using the DX9 binary because I don't really care about DX10/11 features -- I don't think I can even name any DX10/11 features -- but coworkers of mine have gotten really bummed about the game because of this.
- I have a savegame on Steam Cloud where issuing an order to a particular Helicopter Gunship reproducibly crashes the game.  another savegame on Steam Cloud (which I overwrote accidentally) says it's not compatible with your current mod set, no matter what set of mods (including the empty set) I have turned on.  since these are on Steam Cloud, I'm not even sure I'd be able to download these and send them to Firaxis so they could fix them.
- the game seems to crash a lot when I switch into, or out of, strategic mode.
- if you start building a spaceship component, and then switch that city's production away from the spaceship component, you can never build it from any city again, making the science victory impossible.  I just hit this in my game.  it SUCKS. 
 
whew.  it feels good to vent about these things, especially that damn spaceship bug.

just to provide balance, here are some great little things I love about Civ 5:
- you can move tiles between cities.  the fact that cities in previous Civs would get irrevocably attached to tiles really sucked if you had to drop a city to grab a resource long after its neighbors were founded. 
- embarking.  I was inordinately jazzed about this feature when I first saw Civ 5, and it's still pretty great.  so jazzed.  I fist-pumped a little. 
- the list of important shit to do before the end of your turn.  literally the best thing that has been added to Civ, ever.