Nearly the same as IV and most of what they changed was a mistake.
That said, they did change a few things but they were all bad decisions. They got rid of stacking units so now military units cannot occupy the same place. This sounds like a good idea until you realize a single tile which can contain a whole city cannot support more then two small scout units at the same time. As a further example try to move a convoy of ships through the English channel on the World map and you'll find that no more then ONE or TWO ships can move through the WHOLE channel at once. This is a channel wide enough for SEVERAL FLEETS to pass each other with plenty of room but in Civ 5 you have to walk EACH ship individually through the channel.
They've also dispensed with transport ships. Now land units automatically embark into age specific boats when you direct them onto the water. That sounds convenient until you realize that any sizable army is going to be spread all over the place into dozens of vulnerable and completely unprotected boats that can be insta killed by just about anything. In civ 4 you could stack transport ships with escort ships. If the stack was attacked the escort ship fought first. So nothing could hurt the stack unless it first got through the escort. This was a much more logical and manageable system.
Hex movement is fine but it isn't really innovative, interesting, or have significant impact on gameplay. It's there but it's sort of like talking about the shape of device's knobs like anyone gives a damn.
The game seems to have a personality disorder in regards to combat. It provides you all these ways to win or gain power that have nothing to do with combat and then every innovation and tech upgrade seems to be all about weapons. I mean, nearly everything you research directly impacts your combat effectiveness. All the cool toys are weapons or ways to kill people. Which is great... who doesn't like a little war in an empire game. But it's a bit like hanging out with a gun nut that wants to read you love poetry.
Tactical strategy is just as shallow in Civ 5 as in any previous title. A major reason why they wanted to get rid of stacking units was to encourage players to make use of the land for attack and defense. Sadly it has little impact on the outcome of anything and it's contribution to feel of the game is almost nil. It would be nice if instead of negating stacking they could instead allow either a real time or turn based tactics game mode that could be optionally selected when entering combat. You could be presented with a generated map based on the tile, organize your troops, and then either attack or defend as you chose. Thus you could have macro strategy and combat tactics without imposing all the problems that forbidding stacking causes.
Beyond that there really isn't much to say if you've played any previous civ game. There is nothing good about Civ 5 that wasn't done better by previous games. Even the graphics aren't worth it.
Sid, either seriously innovate with the Civ series, do something else, or leave gaming and stop jerking us around. I have loved enjoyed many of your games but honestly the high point of the series remains Alpha Centauri which is still the most innovative Civ game to date. You can do better then "this"... dig up some of that youthful competitive hunger and make a great game - or retire. You can't just turn out a retread of an old series and expect us to swallow it with a smile.