Beside the groundbreaking principle, what's left of Civilization ?
You played one game, you played all.
You could say the same about most games. Particularly the more popular shoot-em-up/beat-em-up types. Few things are as tedious as 'farming' in World of Warcraft, in my experience, for example.
I see no reason why this shouldn't be more and more boring with time.
Really, I've played Settler diff.level Civ5 games lately, it was fun but boring around the end, especially if military is involved. Now I just played a King diff.level game, man, it's boring but from the start ! Everything is so slow... You know you have to build the National College, but you have to build libraries in every of your cities first..
Which is one library if you have one city, two if you have two, seven if you have seven... How many cities do you want when you reach a position where you can build the College? Fewer mean it's cheaper and you need fewer libraries, but at the expense of expanding early. Which is better in your situation? It will, of course, vary with the game, your starting position, your choice of civilization...
you know you have to build colosseums and markets in every of your cities also in order to compete...
No you don't. Happiness buildings affect the whole empire, so it doesn't matter where you build them. How many you build is dictated by how much unhappiness you have - which in turn will vary depending on how many cities you settle, how many different luxuries you can gain access to, which social policies you choose...
A market in a big city with lots of commerce will be as productive as several markets in smaller cities. Which is best for your situation? Do you have a single commerce hub with lots of resources a river? Do you have production-heavy cities with hills and forests that can't produce that much gold, so a market wouldn't be very useful there?
Don't get me wrong, I'm not naive to that point to consider Civilization other than a collection of simplistic, unfun and boring mechanics anymore. Me too I was pleased by the combat animations ! They were very good ! Me too I was pleased by the nice graphics ! But after ? Does it really have just this to offer ?

Well it seems that yes. Civilization 5 is only an update of the original game, as have been Civ2, Civ3, Civ4, etc...
This is why it's called Civilization rather than something else... People tend to cry, even scream, foul when changes even as limited as those between Civ 4 and Civ 5 come along - anything that made it play 'not like Civilization' would be a dealbreaker.
Is Civilization "unfun" or boring? To you apparently, to many others no. This is subjective.
Is Civilization simplistic? Ultimately, yes to some degree. In any essentially single-player strategy game, particularly one like Civ in which interaction with and ability to derail other players' strategies is more constrained than in, say, chess, you are ultimately going to run up against the inevitability of one or two optimal strategies that are always the best route to victory. But it's plain even from your limited testimony that you haven't begun to reach that stage yet - Civilization is somewhat simplistic, but far less so than you're making out. And not all of the national wonders are as necessary as the National College - sometimes you will have situations where you will need to decide whether you should go for a particular one.
So let me ask you something... are you really a fan ? How this can be ? Did you played every iteration of the series and took, objectively or even subjectively, the same pleasure each time ?
Yes indeed - and indeed I can say that having played both Civ IV and Civ V concurrently, so can directly compare my experience with the two over the same time period.
Come on, if you answer 'yes' to this question, I really want you to tell me something : how can this be ?
What do you find in the different iterations of the series that renew your pleasure on intact ? Don't you feel you are playing the same game over and over (beside the little mechanics twits which are lame), without major changes ? What is making your feeling ?
Yes, I feel I'm playing the same game over and over - which is exactly *why* I like each version. Civ V, for instance, is mechanically more different from other games in the series than any of its predecessors have been from one another - however it feels pretty much exactly the same game when playing it. If I didn't want to play the same game over and over I'd only have played the original Civilization once and then never played again. It's a stupid question, I'm afraid - what makes wanting to play Civ IV and Civ V any different from wanting to play Civ IV multiple times? It's the same game, but the gaming experience is different every time.
What games do you like to play? Do you like to play them only once?
As a "Civ fan" myself (whatever it means), but a delusioned one (yes Civilization series exists to be up to date graphically, not to evolve)
This is true of every game franchise in existence. In almost exactly two months Blizzard will be releasing Diablo III, a game which proudly boasts of being nearly identical (or, in marketing terms, of retaining popular elements) to a game that was released at the turn of the century, which itself was nearly identical to a game that was released in the 1990s. They recently released a sequel to Starcraft, another '90s game, and promptly got roundly criticised by members of the fanbase by not making a very similar game similar enough for their tastes - exactly the same criticisms get levelled against Civ V by Civ IV fans here.