Well, by now I would assume that all of the people who like CiV check these forums regularly, and all of the people who don't, well, don't.
So assume a bias towards liking the game.
I like it much much better than 4, for many reasons. Reasons that haven't changed since the game came out. The things that have changed were not issues for me at any point, so my opinion has not changed. So if you thought it wasn't a good game before, I doubt that the patches would have changed it. At the very least, if it did somehow fix what you didn't like, then I would think that you were disliking it for all the wrong reasons in the first place.
So here are the differences between the games that I can think of, and I'll try and provide a short argument as to why CiV is better than CIV in that regard.
Military
CiV: Hexes, 1UPT, no guaranteed death/battle, unit embarkation, fewer units, limited strategic resources, city HP, ranged combat
CIV: SoD, transports, more units
Unless you like Stacks of Doom, CiV is clearly the winner here. And if you like Stacks of Doom, I don't see how anything else could convince you to play 1UPT. CiV has so much more in the way of tactics, but CIV does have the flavor of more units. Which is just clutter from a gameplay perspective.
The military AI is just as abysmal as in CIV, but the mechanics make it much more painful to watch in CiV. Because 1UPT is harder to maneuver troops in and the AI can't just build a bigger Stack of Doom to win, they get crushed often and hard. Once you get good at the game, play against friends.
Diplomacy
CiV: Research Agreements, AI diplomacy not based off of religion only, City-States
CIV: Tech trading, Apostolic Palace, UN Resolutions, espionage
Tech trading is terrible gameplay-wise. And not particularly appropriate flavor-wise. Research Agreements are better for gameplay because they have an opportunity cost (can't buy other things and can't war with that player). It isn't that much better though, they can still be abused.
The City-State mechanic is a great idea, but not at its full potential. It will be much better after G&K, as CS will have more and better quests, influence will be about fulfilling these quests (and being a true ally/friend) instead of only about money, and there will be more types of CS to help more playstyles.
As for the UN, in CiV it is there ONLY for the victory condition. Which actually works with human players. The resolutions were fun and flavorful, and I do miss that. But I thought the civic system was stupid, so those resolutions were meh.
Espionage will be added in G&K, and I thought CIVs espionage system sucked. It can only get better.
The thing people complain a lot about in CiV is the diplomacy AI. As a competitive multiplay player, I appreciate having an AI that is just as ruthless as I am. Though it may seem like they attack you "out of the blue", imagine what you would have done in its place. I certainly wouldn't let on that I am about to declare war. And I would try to convince as many people as possible that the player in first place needs to be ganged up on. Since you pick the difficulty you can usually win on, that person is usually the player, you. So I like the diplomacy AI.
Economy/Culture/Tech
CiV: social policies, road/building maintenance, fewer superfluous techs, global happiness
CIV: civic system, local happiness (and health), many techs with many items, corporations, religion
Again, CIV has CiV beat out in flavor, with more content in the tech tree, a civic system that correlates directly to real-world governments, local happiness, corporations, religion, and a health system. But again, I think with simplicity lies elegance.
Civics vs. Social policies. Civics sucked. It was easy to switch to whatever civic you wanted, there was little to no opportunity cost, and even less planning. With civics, you were never making a single, important decision. You could always edit-undo, but with a small penalty. With CiVs culture system, bonuses stay for the whole game, and you not only have opportunity costs within the system (as did Civics), but within the game as a whole because you need culture yields to get policies. Flavor-wise, social policies don't make as much sense unless you view them as cultural traditions. Most people tend to think of them as government forms, which the Civic system better represented.
Buildings and roads have maintenance costs, creating even more opportunity costs (cant use money for other things if you use it for these) than there were in CIV (not enough hammers/time/workers to build all of them).
Superfluous techs have been removed, which removes a lot of the flavorful content from CIV, but makes the game more concise and less fluff. Each decision is important, and there are not so many decisions that they are unexciting.
Global Happiness may not be particularly realistic, but it again creates interesting decisions. Do I have fewer cities but not have to worry about their happiness (and thus grow them), or do I expand and try to balance things perfectly (which involves keeping population moderate)?
Religion was fun in 4, but looks like it will be much better in G&K for 5. Corporations were religions with a modernist flavor. Eh.
The health system is wholly unnecessary. I am glad it is gone.
Essentially, I would say that CiV is much better for gameplay, whereas CIV is better if you want something along the lines of SimCity (not that SimCity isn't great for gameplay or anything, that just has to be the type of gameplay you want). CIV was much more "this happened in history, let's make a game mechanic to fit it" and CiV is much more "this is a good game mechanic, where can we give it flavor?"
EDIT: In the time I wrote this, there were 4 posts! One from the OP, who reminded me of 2 things that I always forget are missing from CiV because they are there enough.
In base Civ4, many of those tile improvements that were missing in 5 were not there either! And besides, not giving you every option creates important decisions, again. Since on hills I can't choose to build a windmill for food (only farms for food in Civ5), should I found my city in all these hills? I could found 2 small cities, or find a hill with rivers (because you can build farms on those), etc. So no, they haven't been added, and yes, I'm glad they haven't.
As for trade routes, they have always been in CiV? I don't understand. They bring money, movement, cost money (so sometimes they aren't worth it), and give other bonuses depending on your civ/policies.