According to Steam i've played it for a total of 193 hours and unlocked 88 of 140 achievements. However that "140" part is kind of dubious. Last i checked there were 13 achievements that were verified to be bugged, and the current list has 12 or so achievements with titles like "NEW_ACHIEVEMENT_NAME_137_1" :) So that means there's about 25 actual achievements left for me to get, at least a dozen of which should be pretty easy. I'm currently going through finishing up all the leader specific achievements right now.
The addictive "just one more" lure of achievements is certainly enough to keep me coming back to the game for now, but it will be interesting to see what happens when that well has been exhausted.
There's definitely a lot of trivial items where Civ 5 fell down on the job. Civ IV had better intro music, a better intro movie, better voice acting (aka Leonard Nimoy =) better wonder movies, better endings, and better (as in existent) recaps.
(Speaking of which, in a rather strange coincidence, the main Civ 4 song has just been nominated for a Grammy :)
Civ 5 had rather bland and forgettable music for the intro, and the intro movie itself was kinda dumb. The voice acting is good, but it's certainly not Leonard Nimoy. The wonder "movies" are a joke. They're pretty much consist of zooming out of a still picture of the wonder in question. The ending is pretty much just a text box saying "You Won!" Actually, that's not 100% true. If you get the space race victory there's an animation of the space ship tacking off. Which is usually covered up by the text box =P I was surprised to find that despite the initial appearance Civ 5 _does_ in fact create a recap file, the game just doesn't come with any way of viewing it. There's at least one 3rd party piece of software that will let you load the recap and watch it.
All of that sounds pretty trivial, but for me at least it does have a noticeable effect on my enjoyment of the game. Having something cool pop up when you achieve something is addictive. "Just one more tech" or "just one more wonder" is the next level of "just one more turn." At least if the reward is cool. Even if you've seen the movie dozens of times before, even if you decide to cancel out before it's finished playing, it seems much more rewarding to me if you actually like the movie itself. Civ V kind of falls down in that regard.
It's hard to say how the two compare on more fundamental levels however. It's certainly true, as more than one reviewer has said before, that the one big change everyone was talking about before the release, "OMG! They're switching to hexes!" actually turns out to be pretty inconsequential when you're actually playing the game. The one unit per tile change on the other hand _is_ pretty significant. Both because of the changes it brings to combat, and because the AI is pretty poor at handling those changes. As i believe i've already mentioned, this was the first Civ game where i didn't seem to have significant difficulties winning on the hardest difficulty. The thing i miss about Civ 4 the most however is the ability to build really impressive cities. They seem to have made it much harder to build stuff compared to how easy it is to research things. You'll often have a huge backlog of things you want your cities to build that you just don't have time to get around to. Also the food supply has been greatly constrained, so you'll never have a city grow large enough to take advantage of all the tiles it has access to. (One of the several factors that has encouraged the return of Infinite City Sprawl.)
It does seem that the games are starting to get very similar, but that may just be because i need to experiment with play styles a little more. There is a pretty set path you want to follow on the Social Policy track depending on what your strategy is, but at least it's a different path for different strategies. In Civ 4 i was beginning to feel like the early turns were set in stone between trying to monopolize the religions and occasionally going for the Civil Service slingshot. (Although to be fair, i never did get around to trying the expansion packs.)