How does it compare to previous entries?
Never played I and II, just a bit of III but the comparisons are mostly between IV and V right now. The AI apparently sucked in all games, but I've never seen anything as bad as the AI in naval warfare in Civ V. It's so easy it bores me to death.
Civ IV was so much friendlier to mods that it's not even a fair comparison. As for happiness, it used to be city-based, not a universal score.
The main difference that you can see right off the bat between the two is stack of doom x 1 unit per tile. I prefer stacked units because:
1. Regarding realism, stacked units are much closer to actual armies.
2. Moving a stacked group is many times faster than moving 1upt. It basically ruined multiplayer IMO. Whenever you reach industrial/modern era, turns may take up to 20 minutes (even when people use queues).
Even earlier eras in maps larger than standard are insufferable to me. I've had 5 or so friends abandoning Civ V MP simply because turns (and matches in general) would simply take too long and they'd get too bored/pissed off. I still play it, but only as long as the map is standard or smaller.
I know there's PBEM, pitboss, etc, but random people quit the game most of the time and matches can take weeks to finish.
3. Maps can look incredibly silly with the 1upt system. It is simply not possible to create scenarios where you're supposed to be vastly outnumbered in a small area.
Not saying IV had perfect combat by any means, since chance was a big factor and didnt work well at all, but I don't think 1upt is the solution.
. I been on a marathon game in Civ 5 with Shaka. I took over Egypt after a long off and on war with them for years along with the entire world 
