any initial impressions will be most appreciated... thinking about adding this one to my collection as well, but i want to hear from players, as opposed to editors (not that those are mutually exclusive mind you)...
Played most of today (gogo vacation day!). It was rock solid for me, with no crashes, memory leaks (that were apparent after five hours straight, which is when I quit for a break the first time), or even slowdown. Friend of mine did get a few crashes, we think it's a driver issue. Dropping to DX9 seems to fix it for some people.
First off, yes happiness is global now. Population, more cities, and things like annexing cities through conquest increase it. You can create a "puppet" government on conquest that removes the extra unhappiness penalty from conquest, but you also can't directly control the city if you do that. It will give you it's generated science/gold/culture, but it builds what it wants and never builds units. You can choose to annex it later if you want to. Ghandi's unique ability is REALLY strong (double unhappiness from cities, but half from population), as the population modifiers get far bigger then the city ones if you have big cities.
If your empire is collectively unhappy, you get a growth penalty. If you're REALLY unhappy, you get a millitary penalty. If you're happy, you build up to a golden age. I never found managing happiness per city to be interesting in Civ 4 because it really only mattered in big cities, so I'm fine with the new system. YMMV.
The other thing I found is that the pace of building things feels slower. It seems like building high production cities is harder then it was in Civ 4, and some of the units and improvements take a long time to build. The downside is that the pacing feels slower. The upside is that it means if you neglect things for a while, you'll be spending a lot of turns (or a lot of gold) to compensate. City borders also grow more slowly, further adding to the feel that the pacing is slower.
Gold is a LOT more important! Gold lets you grow your borders faster, instantly build almost any unit/building, and build/maintain City State relations. You no longer need a civic or ability to buy buildings, and if you want to you can pour your gold into a city and build it up in a hurry. You can also get millitary units in a hurry if you need to. Gold can't be spent on science directly, but you need gold to set up research pacts with other Civs and that helps you get tech faster.
City States are probably the most interesting new thing. They're the s*#@ disturbers. They might want you to build a wonder, or get a resource, or give them units... or take out another city state. Do what they want, and they will give you significant bonuses (access to resources, and one of food/culture/free units depending on the type of city state). Relations slowly decay but you can either do something else for them or prop them up with gold. Other Civs will sometimes ally with them too, or try to conquer them. If that happens they'll ask for help, and if you liberate them you become their friend for a very long time. You can also just conquer them yourself, but I found it more worthwhile to make friends (particularly with the Patronage social policy tree).
Social Policies are both interesting, and annoying. It's interesting because there's a lot more choices then with the old civics system, and you won't endure anarchy just to fip back and forth between theocracy and organized religion depending on war or peace. But it's also annoying in how slow they are to get until you start building up a lot of culture production, which takes quite a while. I hope they tweak the costs downward a bit. Here's a list of all the policies and what they do.
Combat is easily the most improved part of the game over Civ 4. As opposed to the stack of doom and one warrior somehow being able to conquer a city of 5 million people, combat now plays like a strategy board game where the cities aren't so helpless. It's not like cities are invincible fortresses that someone suggested, far from it. But a size 10 city with walls, a castle, and a garrisoned defender is not going to fall to a single Knight.
The combat itself is a lot more interesting now that positioning actually matters. You need to keep a Great General with your army if you have one, but not right upfront because he can be attacked (I tended to stack them with the siege weapons, as their radius of effect reaches a couple hexes out). Siege weapons require a move to "set up" and deploy before you can fire them, but cannons are as lethal as you expect when shooting at ranks of pikemen. 
In general ranged units can win the day if you're guarding them, but if you let a melee unit get a run at any ranged unit they'll get completely destroyed in all but the the most ridiculously lopsided mismatches (industrial era Artillery will probably survive an attack from a Swordsman, but that Swordsman will chew through a Catapult or an Archer like it's nothing).
The other neat thing is that some units (and some buildings!) require strategic resources, which are limited. 4 iron means you can make 4 units that require iron. Want more? Either get more iron, or get rid of something. Factories work the same way with coal resources. In my game the city states mostly had Iron so when I managed to befriend four of them at once I had all that I needed, but without that I'd have been fairly constrained. Late game units are particularly bottlenecked by Aluminum, so you'll have to balance your air superiority fighters vs other requirements with some thought.
I found a couple minor bugs. Stuff like if you liberate a city from a Civ who had been eliminated (and thus bring them back into the game, which you can do now) you might get status messages that don't show that Civ's name in the label. Stuff like " has made peace with Alexander." Pretty minor, but amusing. Also, the time between turns is longer now in the early game, but I didn't find it getting worse at the same rate it does in Civ 4 (probably because the AI can't stack 15 units in every city).
All told, I like it. Some of the changes work better then others, but it's really fun and the time put in to getting it right before release shows. For anybody who is sick of sequels that are the exact same thing over and over again only with different maps... well, let's just say nobody can accuse Firaxis of that. 
edit - Oh! The UI is really good. The status mesages now appear as icons on the right side that you can deal with when you want, there's almost no annoying popups (Wonder completion messages and diplomatic requests still do, but events and the city queue don't). The tooltips on unit commands are quite good, and you can figure out what almost every command does just by reading the tooltip for it. In the case of workers it gives you recommendations that are usually pretty good, as well. The combat odds shown are now also actually accurate, but combat being able to end with both units surviving probably helped to make that easier.
