In response to the OP:
The following is based on only limited time playing CivV. I have experimented while staying at a friends house for a couple of days.
1) A lot of bells and whistles have been removed, e.g:
- espionage
- religion
- corporations
- vassal states
- random events
These are features that you could pay only a small amount of attention to while play through a game of CivIV, or you could base your game plan on them. The advanced start up menu even allowed you to switch off these game elements.
2) Elements of basic game play that were present in the Civ series as far back as CivII, if not earlier, have received significant overhauls.
- empire happiness has replaced the happiness of individual cities.
- hexes have replaced squares, and the fat city cross is gone.
- In previous games each of your cities' income was divided into science, gold, and luxuries/culture. The split was determined by a single empire-wide ratio determined by the player.
- only one unit can occupy any given tile. Previously there was no limit.
While these have all taken away options that were standard throughout the last few iterations of Civ, I'm not yet convinced that the amount of strategy has been reduced significantly, or at all.
3) CivIV BTS had, thanks in no small part to the customisability of CivIV that also exists in CivV, a rather good AI. CivV does not.
My own opinion is that, ignoring some bugs representing obviously poor behaviour, this is as a result of:
a) changes to core rules. As more and more people put in more game hours of CivV, best strategies can be determined and the AI can be adjusted to adopt these strategies.
b) the BetterAI mod began life as an attempt by a player to improve the CivIV AI. It dramatically improved the AI, so much so, that the first official patch for the Warlords AI incorporated an early version of this mod. CivV has not existed long enough for such advances to have been made.