My post is not just empty praise, and may have some criticisms included, but they are considered ones for the context of civ5, and please accept that some comparisons to civ4 are inevitable.
-Maps seem to be very large. In all the streamlining that's been going on, at least we haven't ended up with the mapsizes that were typical in civrev.
Even the default small map has impressed me.
-Modding potential. It looks like this will already be more moddable than civ4, and even accessible to more novice modders (which most believe is a good thing). In civ4, I felt that with the SDK I had the power to change almost anything about the game I wanted, though I don't usually have the know-how.
I have great optimism for what mods will do with civ5.
-If a unit has yet to receive orders, I like that the "unit needs orders" button prevents you from ending the turn early, though it is a tad laggy at times. (By the way, for those who don't know, Shift+Enter can still be used to skip unit orders and force end-turn, if allowed)
-Civ5 introduces more incentives to explore, and also to get that exploring done earlier. City states give more gold if you're the first to find them, and natural wonders add to the happiness of your empire, even if you're not the first civ to discover them *looks angrily at civrev*. Also, you're required to do exploring yourself a bit more because there is not tech trading anymore (though you could automate explorers if you really wish). I may eventually change my mind about this, but at this point I like that you have to discover the world yourself rather than just have some point in the game where "boom, you just paid some gold to uncover all the tiles in the world".
Also, the greater importance on exploration means the scout is more often a unit that is worth building (but not by a long way, I admit).
-More frequent / easier to obtain golden ages. To add, it's nice that having excess happiness actually contributes to something. Even if it's not the most useful way to use excess happiness (i.e. one should expand/grow instead), it's nice that there is at least something positive about the difference from 0. On the other hand, it would be nice if the penalty for a very unhappy empire actually scaled with the size of the unhappiness too, because once you go past that threshold (-10) you begin to no longer care and just go on your way conquering the world but with no population growth and the small inconvenience of a combat penalty.
-Ability to "lock in" citizens to particular tiles while using the citizen automation and emphasis buttons. Now you can ensure that gold mine gets worked even if you tell the city to emphasise food. It's not quite perfect, but it is so much easier to use than civ4's city governors IMO.
-No more prebuilding improvements on top of existing ones. Now you actually have to make the sacrifice of destroying an improvement in order to begin a new one, and you can't even 1-turn build it either (by using multiple workers). I've found this has reduced my worker micro quite a lot, because I'm pretty much OCD about these things
, but sadly it's still possible to get some micro advantages out of pre-builds when there aren't existing improvements on a tile.
-I love that there are no more civ4-style (and previous civs IIRC) granaries where its effect was to store one half of the food for next growth (though the hospital now does it instead, but it's a late-game building and expensive to build/maintain so it has much less impact on gameplay). The building was just too powerful and dominated pretty much every build order. It also lead to not-so-intuitive ways to make use of the whip and finessing with the angry-countdown timer.
While some people appear disappointed with how narrow-focused some of the buildings are now, I appreciate that fewer of them are so obviously overpowered anymore, like the library and granary.
-Cities having 3 tile radius. This gives you more flexibility (at least on the surface it appears to) in tailoring cities to just the right size you want. It's for all intents and purposes impossible to fill up the entire city radius with citizens from a single city in the course of a normal game, so there's much less sacrifice in placing cities with overlapping BFCs and IMO the decisions about where to settle are equally, if not slightly more, interesting as they were in civ4. People have been complaining about how weak the bonus resources are, but this is not a bother if one tries to get used to more reasonable yield levels rather than the pretty extreme cases of civ4's river-side grass corn, or the copper mines.
-I'm sure it's been mentioned lots of times, but finite strategic resources. These resources seem a squidgeon too abundant on some of the mapscripts, but at leas this can be adjusted via the mapscripts themselves. Now in civ5, attaining more copies of a strategic resource gives you a greater benefit than simply having one copy of it, so there is incentive to go out claiming or capturing more.
-Jungles are a bit better now, with an interesting bonus given by a building (universities). In civ4, there was pretty much no reason to ever keep them because they gave a health penalty and they prevented access to a strong grassland tile.
I could add to the list later, but these are the things I have specifically noted so far, and that may have received not much mention yet.
Thanks for reading.