A couple notes:
C4: during the earliest expansion you had to fight of animals everywhere. Then the barbarians came in droves & you better have been prepared! Even building improvements was often a chore because of barbarian activity. Meanwhile, while trying like hell to keep a footing, you were trying to create enough wealth to create enough research to make strategic bee-lines to pivotal wonders or to found a religion.
I remember going out on 40 to 50 turn limbs to get Code of Laws. Oh and the agony of waiting all those turns and hoping your frontier troops could hold off the barbs just long enough -- I'm breaking into a cold sweat now.

If you were fortunate enough to get an early religion, then you had many different options to juggle -- gotta spread the religion, but can't neglect everything else......this was trade-off decision-making & rewarding when you made the right calls. Meanwhile, with everything else demanding attention, espionage also required at least some attention. There were just so many items vying for the player's attention in C4 -- and to neglect any one thing could create real problems later.
By contrast, in C5 it seems there are just fewer decisions to make and the decisions seem to carry less weight. Example: In C5, diplomatic decisions have consequences, but they seem less meaningful because the consequence aren't very clear. In C4, building relationships required conscious efforts -- sometimes you had to swallow real hard and make a really difficult decision regarding friend or foe -- because in C4 you knew that these decisions had lasting and substantive consequences.
Armies were bigger in C4 and they could be constructed quickly. Your weak and passive neighbor might quickly chop out a few units, enslave a few more, and suddenly within a few turns be ready to knock down your door. In C5, armies are smaller and built slower -- you don't have to keep a close eye on your weak neighbor because you know he can't just chop and enslave his way to superiority in a matter of turns.
All local politics is global in C5. So, if I conquer a new city and bring 3 unhappy citizens into my empire thereby -- no problem -- I'll build a circus in some other city to somehow appease those citizens. And anyways, the negative consequences for unhappiness only matters when it reaches specified and arbitrary thresholds -- there's no difference between 1 unhappy and 3 unhappy citizens. Also, the healthiness of cities is no longer a concern the player no longer has the tough call to make between health and happiness.
Buildings take so long to build they almost feel like mini-wonders by the time they're built, but their effects are somewhat underwhelming unless you're stacking multipliers by specializing your cities (which I always carefully do now). OTOH, I have noticed that if you build the granaries and watermills early, and shift as much over into production as you can (even sometimes starving the city) when you're building the big stuff, then production times are actually pretty comparable to C4.
The tech tree has been simplified. Now, fewer techs do more things. I'd prefer more techs doing less things, which would require tough decisions and trade-offs. Research agreements are a passive process that doesn't seem compelling. Units don't require pre-reqs that makes sense as before.
In short, it just seems there's less going on in this world and what is going on is slower paced and not nearly so compelling. I'm just trying to give words to how I feel when I play C5 v. C4. Now, having said all that, I actually am enjoying C5 for the first time with this latest patch and 5 mods thrown in. So, things are looking up as far as I'm concerned.