Carcassonne is one of the most successful eurogames out there, having sold millions and millions of copies (while most titles would feel lucky to break the 50,000 mark), winning the 2000 spiel des jahrers and the hearts and minds of many boardgame enthusiasts and casual gamers out there. With just a few matches of it, the reasons why this happened become quite clear. I don't mean to say that sales equals quality, or that only good games sell well; I like too many (really) obscure games and dislike a good number of popular titles to be able to say this. What I really mean is that this game has quite the wide appeal, being able to please both the more casual side of gaming as well as the more focused, “serious” gamers. This review is kind of a list of qualities that I think made the difference with this game.
Visuals
This game has a ton of visual appeal. It's not luxurious by any means (it has to keep that price tag down, you know, to seel those millions of copies), and it doesn't have anything like beautiful art, that you'd hang on your wall. It's just a great looking game, in a rather simple style. The player tokens have a lot of charm, to the point that the meeple (mini-people) have become a sort of symbol to the entire hobby (or at least the eurogame genre). Also, there's an undeniable and somewhat unexplainable pleasure in seeing the map develop as players lay their tiles and cultivate their cities/streets. Every game ends up with a different map, and although it has nothing to do with the actual gameplay/strategy of it, it's still nice to see the visual end result of the 60 minutes spent playing the game, the players do get a feeling of building something up together.
Easy to learn
Rules are rather simple, and to a point rather intuitive. Your turn: you draw a tile and you place it on the board, choosing if you want to place a meeple on one of its features or not. It may take a few plays to really see all the possible placements in your turn, but it's rather intuitive. The farmer scoring is probably the only exception to that, with the more recent editions including a player aid that exclusively deals with this rule, but still, it's a game that can be explained in 5 minutes at the most. If you compare it to dominoes (which is a rather good comparison), it'll make it even easier for the newcomer to understand and get started.

Depth
Like I said in the previous point, it takes a few plays to see all the strategic possibilities one has during your turn. For this reason, an experienced player will probably always beat a newbie, even with the undeniable luck factor of the tile draw. True masters will probably memorize the tile distribution of the game, and build according to the probabilities of specific variations coming out; more casual gamers will still take pleasure in seeing their capacity of developing strategies and different plays evolve naturally with each game. You'll quickly (or maybe not quickly enough) figure out what placements are risky and which ones are safe.

Variety
The game scales from two to five players, something that all game publishers push their designers to do. But more than that, the game manages to vary between destruction and cooperation, gamer's-game and family-game. One can play this very agressively, stealing oponnent's cities and laying tiles so that it makes their oponnent's constructions less likely to be finished, or you can just “mind your own bussiness” or even cooperate, with two players building up the same city so that they develop a nice margin towards the other oponnents (just make sure you don't forget the possibility that your “partner” may try to take the city for himself at the end, with an extra meeple placement)
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Modular aspect:
As one can see from just browsing online or a local shop's shelf, it's a game system that lends itself to quite a bit of expansions (half a dozen, up to now) and variations (two of them). I don't know how casual gamer sees this expansion phenomena, but I'm sure it doesn't hurt the game, even if some (or perhaps most) of the expansions are somewhat artificial. I recommend the first two expansions (Inns & Cathedrals and Traders & Builders) for those who want a little more going on in their game, but be aware that the blocking strategies will suffer a bit with the new tile designs (some of them rather crazy). The other ones, from what I've heard and seen, are not so hot, and should be pursued only by completists or those who seek extensive changes to the game's mechanisms.
While Carcassonne to me is just “a rather good game” (far from being one of my favorites), its success is definitely understandable. Me, being the agressive type, much prefer the game with fewer players (2 is the best, with 3 being the maximum for me) with more blocking and stealing.. With a more experienced oponnent, I even suggest a variant that has the players keep a hand of 3 tiles at all times, instead of just being forced to play what you get that turn. With this variant, I rate it a solid 8 out of 10; without it, it's more like a 7/10.
I'm a rather picky game owner (I hesitate little before selling a game that's “just ok”) and Carcassonne still manages to have its safe place in my collection. It took a few plays to get the depth of it (since the visual appeal and the building aspect were not enough for me), but now it's a game I quite enjoy playing. I've had around 15 to 20 matches of these (more than half without any expansions) and I'm sure I'll enjoy plenty more, it's quite replayable due to the variety of the tile draw, even if there are some clear patterns in the development of each match.
But even with all its success, Carcassonne cannot please everyone (no game can, after all): those who severely dislike abstract games, or perhaps more accurately, games with abstract-like gameplay, they will not enjoy carcassonne. People who dislike confrontational games should avoid playing with experienced players (a.k.a pretty much everyone that's playing online). But I'd guess that this would be a fine fist-step for people who want to get into the hobby with their families (the 8+ age estimate is accurate, for casual play), or adopt it as a light-hearted experience, something to play while having a few beers and chatting with friends. It's a quick game, around 60 minutes or so (more, for those who spend a lot of time thinking on their turn), and we usually fit it in between two or more substantial games in our gaming get-togethers.
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