Areas where Civ 5 improves over Civ4:
1 unit per tile
How units can embark on the sea without a transport. Makes navies much more valuable.
Areas where Civ4 is better:
I disagree, there's a number of things that I prefer about Civ5.
I prefer the way culture and borders work in Civ5, they don't balloon out every couple of centuries, borders trickle out one tile at a time making border shapes more interesting. Acquiring tiles of rival civs is no longer passive with a small diplomatic hit, it's an aggressive act that can lead to war, which is more realistic.
I like that roads cost money to maintain, it makes planning your road network more strategic. Building a road between two outlying cities may be useful if an enemy invades, but is it worth the cost?
City states add variety and character to the game, previous Civ games seem lonely without them. It's true that in the vanilla game they were basic, but in the expansion the influence of money has been greatly reduced and alliances with city states are based on the quests they give you and rigging elections with your spies.
Religion is considerably better in G&K than in Civ4. For one thing, it's not tied to the tech tree any more, it's its own thing. The benefits religions offer are varied and are very helpful if used right. The way religion spreads is also more interesting, it's not so much about how many cities your religion has spread to as it is about how many followers your religion has in each city.
There are things I miss from earlier games, of course.
Localised happiness just made more sense, and it required you to address each city's needs individually. Being able to offset the unhappiness of a captured city by putting a theatre in your capital makes no sense, and it makes rebellions less likely.
Puppet states are a poor replacement for vassal states. The great benefit of vassal-states was that they maintained their culture and their own military, but that was also a danger as if they rebelled against you they posed a significant threat.
I loved Civ4BTS, it's a really complex strategy game with more elements to it than I'll ever fully understand. But I really like the direction they've taken with Civ5, especially with G&K, it may lack some of the complexity of Civ4 but some of the fundamental changes to the gameplay really appeal to me.