As a number of people have pointed out, Civ5 is easier than previous versions. IMHO, the primary reason for that is the combat. I've found that it is a whole lot easier to kick tail in Civ5 than it was in Civ4. The problem lies with the new combat system. I hated the old stacks of doom system. I prefer Civ5's system, but it requires a bit of tactical thinking. Unfortunately, it would be incredibly difficult to come up with an AI that could come anywhere close to competing with a human player. In Civ4, it wasn't so hard. They just had to program it to assemble a huge stack of units and head for your nearest weak city.
There are probably a few things that could be done to improve the AI. One thing is that they are too defensive. I've noticed that, when you get too many units close to their border, they warn you. I choose the reply that says my units are just passing by. No matter what you answer, why don't they send a couple of available units toward one of your cities, forcing you siphon off some units? Heck, when I do an early rush, I don't leave a unit to defend the cities that I have turned into puppets. I just have a horseman sitting near them, in case a stray unit comes their way. Otherwise I rely on the city to defend itself. I haven't yet seen the AI effectively exploit that.
BTW, these remarks apply to the King level.
There are probably a few things that could be done to improve the AI. One thing is that they are too defensive. I've noticed that, when you get too many units close to their border, they warn you. I choose the reply that says my units are just passing by. No matter what you answer, why don't they send a couple of available units toward one of your cities, forcing you siphon off some units? Heck, when I do an early rush, I don't leave a unit to defend the cities that I have turned into puppets. I just have a horseman sitting near them, in case a stray unit comes their way. Otherwise I rely on the city to defend itself. I haven't yet seen the AI effectively exploit that.
BTW, these remarks apply to the King level.