Been playing a bit more lately, I kind of gave up on it too easily when I tried it before.
Once you get involved with it, there area lot of improvements; the interface is much more slick, the graphics are excellent, the combat system with 1 unit per tile does actually make you think about movement more (although I still think there should be the ability to at least stack a limited number).
But every so often I find things that infuriate me. Cant end a turn until you've given every unit some orders; so I have to go through all the units and give them orders (or set them to sleep). Then I click "end turn" and find there's another few units that need orders as well.
Upgrading units - annoying. No option to upgrade every unit of the same type. No option to upgrade to the latest tech unit available. And if you upgrade a unit in a city, it de-garrisons the unit, and you cant re-garrison them immediately. Not a huge deal you'd think, except that garrisoned units create happiness, so if you go through and upgrade your city garrisons, you suddenly find your empire in a state of discontent.
Zones of control - a game mechanic that was taken away after Civ2 and I was glad to see the back of it. But it's returned in Civ5. Just don't see the value in it and it's frustrating not being able to move a unit because the enemy is standing in the next tile.
Happiness - Not sure making it a global resource is a good thing.
Bi-polar AI - You can spend hundreds of game years building a good relationship with a leader, only to have them turn on you and denounce you for very tenuous reasons. Or declare war without provocation. Just feels very disjointed.
Worst of all, there's the AI, which seems to have no idea about combat and I hear it's completely unable to embark land units.
I'm tempted to get the expansion if I can for a good price (I only paid £7.50 for Civ5 itself). Having religion and espionage back might rejuvenate the depth of the game some, but I have a feeling the core issues are going to remain.