I've never really played Civ as a simulation... always seen it as more of a board game. Or at least a simulation with game like rules. I started with The original Civ, and I-IV I found to be satisfying because they continually built on the previous title. Now by the time we get to Civ IV: BTS, we have changeable government policies, religion, a huge variety of units and buildings and mods that add even more to that depth and complexity.
Sure, there are other games (such as the Europa or Crusader Kings series) for greater depth, and I play other series, but I feel as though rather than continuing on that path of going deeper, CiV veered upwards and is now skimming the surface. The mechanics seem overly simplified, you can't stack units, there's no religion, gone are changeable policies, instead replaced with policies that stay with you for all time (which is ludicrous, no nation ever makes a decision that is then utterly unchangeable forever), so on and so forth. I tried a few mods but didn't find anything that gave me that oompf that kept me playing turn after turn with other Civ titles.
So I guess that's what I'm looking for... it's hard to describe other than saying depth and complexity, but I know that there are quite a few people who, like me, complained that CiV was a dumbed down version of the previous titles. Which is just my (or our) humble opinion, not saying it is definitive, but I guess I'm wondering if anyone felt this but now feel that with G&K and/or mods that CiV is now worth the time investment.