Bah! I hate this kind of thread but can't keep myself from posting anyway.
ICS. It should not be the optimal strategy. Fix it. No hard feelings from me.
It took me a little while to warm to civ5. The reason came down to this observation by Valkrion (which I paraphrase and add my own observations here): In civ4, you have a lot of little decisions to make that keep you busy every turn. They are mostly of little consequence and can be corrected later. (E.g., move the slider one turn. Too much? Move it back the next turn. There are many more examples like this.) In contrast, in civ5 you have many fewer decisions to make and the effects don't always manifest for a while. At first, I felt like I was just building stuff with no feel for what was going on. Now that I've played a bit, I spend a lot of time thinking about whether to push culture, gold, research, production or growth in a particular city. If I decide incorrectly, then I will be paying later (literally paying, e.g., for tiles because I underestimated the importance of culture). Now that I can relate the bad effects to my prior decisions, my enjoyment in vanila civ5 has grown a lot, and I'd say that (at least at this moment) it feels deeper than civ4.
I should also say that, after some REXing and near (but not quite) ICSing, I'm back to my more preferred civ style of really focusing on (and specializing) 5 or 6 cities. Notwithstanding the ICS problem (which does need to be fixed), I can say that my preferred 5-6 city approach works fine and is just as doable in civ5 as it was in civ4 (I tend to play on Emperor to Immortal in both, depending on whether I want to win 90% or get my but kicked more often). It took a while to learn to do this in civ5, probably longer than it took me to learn in civ4 (which was closer to previous version), but I learned it. So now I can make my 6 cities and win space race in 1966 on Emperor, which is pretty darn close to what I would do in civ4.
Now, I still think ICS-as-optimal-strategy needs to be fixed. It's silly if this strategy can get you space victory in 1200 (as some claim -- I don't have the wherewithal to test this). But it really isn't taking much away from my enjoyment at this time.