Personally I believe the inability to have greater than one military unit per tile keeps civilizations' military might from getting way out of control. Especially when they've made certain strategic resources NOT unlimited. For example, a certain mine (you know, a certain spot on the map of that resource) for Iron or something, will yield a "number". It takes one iron for each swordsman, for example. I think this keeps the game more realistic in that sense. Yeah, that was a good thing. Also, when you seize a city, you can not only burn baby burn or capture it, but you can also "puppet state" it. This creates less "unhappiness" levels in your civilization. You see, if you outright seize the city, happiness goes down. And I think cooperation with other civs is a little more integral, whether you have vanilla Civ V only, or one or both of the expansion packs.
They removed religion, but put it back in Gods and Kings (and a little bit more so in Brave New World). But they did it better than it was before, is the thing. I can't quite describe it but it's....well, better. Because certain "beliefs" of a religion you happen to have founded cause certain positive things to happen (like, 10% extra growth or something). You start out founding a primitive pantheon of some sort before it moves onto something more specific. But if you founded the religion, then you get to choose its "founder belief" and "follower belief" and, should the game feel so inclined given your cultural level, allow you to add beliefs...and they all have certain positive or not so positive effects on your civilization like I said above.
Vanilla Civ V is not bad, even though it doesn't have some of the neat features. But there is still the choice of social policies...only it's done much better in my opinion than in IV. I wish I knew how to describe it exactly but I can't think of examples right now. Like, you can choose "Honor", which will give your military a boost. Or, "liberty" which speeds the training of settlers and border expansion.
The city states are all right, they do add a little random element into the mix.
I can't think of Vanilla Civ V entirely, as I am now used to it with the expansion packs. Also, some people online via steam have made downloadable content, including scenarios, new leaders...one dude from Egypt had "Days of Sadat" which added Richard Nixon, Jimmy Carter (for the US obviously) and Anwar Sadat and Nasser (Egypt) as well as Golda Meir (apparently it added Israel as a Civ I think). I have never used the amateur-made DLC but the expansion packs were well worth it.
Ask me something more specific though, and I will try to write it down while I'm in a game, which I am going to actually do in a few.
I think Steam, by the way, is a great way to play over the net with others. A friend and I did a game that way though we didn't finish it. It was a little awkward, but maybe the glitches were due to the fact that my friend didn't have the two expansion packs in the game and I do? Hmmm....still, a great way to access the community of players than through the regular message boards online for 2K games. Also, it has a very helpful player's manual there, if I'm not mistaken, in addition to the regular "civilopedia" within the game.
Yes, graphics way better. In fact, the minimum suggested specs involve a quad core processor. You can get away with a dual core, of a certain standard, but it will take a helluva lot longer to load and do other stuff if you've below the minimum recommended requirements. Of course, I have an Oct core and a really kickass video card (don't ask me which one...but I bought it specifically to make playing Civ V easier and prettier LOL).
Since I'm such a p******, I keep it on level three...maybe four, for difficulty. Hey, I don't like games that are impossible to win after all! But I guess I"ll eventually get higher as I get more familiar.
Oh yeah, and there is this nifty as heck "World Congress" where you, or another civ, proposes a motion and all players vote on it a couple of turns later. But that's exclusively BNW content, not the vanilla version.
Final verdict: even if you don't get the expansion packs it is still a vast improvement in some ways over Civ IV. Not that Civ IV wasn't fun! But this is great, too. Steam actually allows it to download any patches that had to be made over time, or anything new the designers came up with. Personally I like how it is integrated to work with steam!