To the OP: Here's why I think Civ 5 is a great game (so far -- I'm still on my first playthrough so who knows what I will encounter in the 2nd half of this game)
--- There's a much greater sense of a story unfolding than there was in Civ IV. In Civ IV, I found that the AI rarely fought amongst themselves, or if they did, they were often pointless skirmishes that didn't result in any real shift in the balance of power, just a mutual squandering of resources. In Civ V, the wars amongst the AI powers are much more dramatic and decisive, and the city-states also tend to get caught up in (and gobbled up by) these roiling conflicts. In my current game, my neighbor Russia is behind me technologically, but is double my size as a result of mercilessly crushing every city-state it could lay its hands on. Meanwhile, across the sea, the Iroquois in a quick, brutal war wiped out the larger Songhai to make themselves into the world's only superpower, 3 times my size and maybe unstoppable. In a way these situations take me back to the original Civilization (Civ I, or whatever you want to call it), where it was a real challenge to win, not so much because the AI was clever or could beat you in a face-to-face even match, but simply because in a survival-of-the-fittest contest of randomly evolving empires, one of them was usually going to draw a strong enough hand to turn into a powerful if clumsy nemesis. Compared to playing Civ IV, sailing around the world the first time and seeing all the turmoil already in action, running into city-states whose greeting is, "Help!" right before they get annihilated, watching from the sea as one army rolls over a whole continent whose interior you've never seen -- it's all very lively and compelling compared to Civ IV, which to me was often more like a still life of a bowl of fruit, diplomacy-wise.
--- I really, really like the Social Policy system that they've replaced the old Civics with. With the Civics, depending on your style of play, there was usually a combination you gravitated towards in every game, wearing a rut through the annals of history as you replayed the same strategy over and over again. In Civ 5, the social policies are far too numerous to grab them all, as slowly as the culture that buys them fills up, so you have to make hard, strategic choices about which direction to take things. Perhaps my favorite thing about the Social Policies is the way they set it up so you have to spend a choice just to open up a new branch to access the policies therein. This makes it most efficient to completely exploit just a couple of the social policy branches. (Conversely, it'd be terribly inefficient to pick a single policy from each branch, as you'd essentially pay double for each choice, the first expenditure being to open up each new branch.) Since the various branches are pretty equally compelling depending on the situation, there's some real strategy involved, and the cost of opening up a new branch of the tree forces you to think ahead to what you are going to want access to in several hundred years' time. There's no more running your government as a libertine, market economy right up until you need to convert your entire society into a fascist police state to support the conquest of the world you've been dreaming about for 2,000 years. Now, you have to pay your dues on the way up to have access to all the military advantages that you'll want for that big war, and to get them you're going to have to skip a lot, maybe most, of the more peaceful, wealth-and-science-creating choices.
--- I love the way cities expand a single hex at a time now, as well as the way you can buy hexes that you've got your eye on. Countries are now far more organic and interesting-looking, and like in the real world, their shapes have meaning -- we're building down the river to control all the fertile land, or we're growing out into those mountains with silver veins running through them. And, the fact that cities can work irregular shaped territory frees up the placement of cities. You still are cognizant of how close the nearest cities are, but it's not as dogmatic of a decision as it used to be, counting tiles over to figure out how far away you needed to be to have the minimum overlap. For example, I was able to build my third city, somewhat realistically, on the coast, knowing that I could grow inland to grab the resources I needed, while maintaining a harbor to build my first navy with access to the eastern oceans. In Civ IV had I built the city there, I'd have permanently lost access to some valuable resources that fell in between two cities.
--- I really, really love having to pay upkeep on roads, forcing you to only build roads you absolutely need. This is a huge improvement over the spaghetti mess that all previous versions of Civ encouraged when it comes to road building. Not only does the map look a heck of a lot better, but the sparse road networks become important strategic objectives, allowing you to fight to keep your one road open in order to allow movement of your troops to the front lines.
What don't I like? I don't like how sluggish it runs on my decent-but-not-great computer. (i.e. GeForce GTS 250 vid card). Since this machine runs Starcraft 2 like a dream, I would have expected it to be easily adequate for a Civilization game, a franchise that has always been democratically accessible to those with aging computers, laptops, or trying perhaps to run under Boot Camp on a Mac with a less-than-stellar video card. Instead, Civ 5 is a brutal resource hog. With moderate video quality settings it sometimes takes 2 to 3 seconds for the textures to fill in on the screen. I can live with it but if you are hesitant about buying it you should have realistic expectations about performance. I may decide to buy a better video card to try and help it along. I never thought I'd be driven to upgrade my video card to play Civilization.
I'm also slightly irked at some of the weird little bugs and stuff I've run into. I guess it's to be expected but the stuff I've encountered seems worse than anything I've seen in the Civ series at least since CivNet.
I'm confident that these things will be fixed in a patch, and anyway I'm having more fun with Civ 5 right now than I have with any game in quite a few long years. The worst case scenario is the longer I'll play it, the more issues I'll find with it, until I don't dig it anymore. But even if that happens, to me it's very much worth 50 bucks to be enjoying it right now for what it is, which is a very fresh and interesting take on Civilization.