I'll be fair: Civ V is not a "bad game." Compared to Civ IV, yes, I would say it is inferior, but standing alone it's not "bad."
I think the most glaring problem is the concept of "Global Happiness." In Civ IV, in order to control City growth and also to make sure Empires did not ruthlessly expand like in Civ III, they had two major mechanics.
The first was city-based Happiness and Healthiness. Each city had its own Happiness and Healthiness. If your city was too big and you did not manage it well, too many citizens would become unhappy and your city would be unproductive. If your unhealthiness outweighed your healthiness, your citizens would become sick and your city's growth would grind to a halt.
On an Empire-wide scale, there was Maintenance. Each city had a Maintenance cost based primarily on how many cities you had total and how far away that particular city was from your capital. This way you couldn't expand as much as you wanted without developing your cities, because your economy would crash.
These two mechanics encouraged players to build up and develop their cities and infrastructure (worker improvements) rather than expand constantly. And it worked pretty well.
Fast-forward to Civ V. For some inexplicable reason, they decided to completely do away with these two mechanics and instead replace them with one: Global Happiness. Now, building a Circus in one city apparently increases Happiness in a city on the other side of the world. Apart from being unrealistic, this means you can have a few cities pump out happiness buildings and not even worry about the rest. Also, if your empire is unhappy, it's supposed to have negative effects, but I've never really noticed it much, and there's overall there's not much stopping you from expanding at your whim.
Taking away city-based happiness and healthiness and maintenance makes the game less interesting, I think. It results in less choices to make, and you rely less on smart management of your empire. It makes Civ V less intellectually stimulating for me, and it's boring.
Now Civ V players talk about building "Wide" or "Tall" Empires. A Wide empire is very spread out, with much less city and infrastructure development, whereas Tall empires are very built up, but not very big. Social Policies are probably the best improvement over Civ IV, BUT their cost increases for city you build, so Tall Empires won't get many. So, you can either A) not expand, which isn't fun, or B) Expand and not get Social policies or build up your empire much, which also isn't fun. It's a lose-lose situation.
I could go on and on, like how sometime I can't build a city on the coast because you need to build cities FOUR TILES APART, but I've already rambled enough. It's really a case of them trying to fix something that ain't broken. It's an old engineering motto: If it ain't broke, don't fix it. And that's exactly what they tried to do in Civ V.