Ok there are a few reason's it made me think of a kind of super-sized, slightly more complex version of Catan.
The hex tiles are the big obvious thing, but the trade of luxury resources (Pearl for Incense, Gold for Wine, etc.) with AI leaders is a similarity because it's repetative. For a whole game I basically traded the same things with the same 3 Civs every 30 turns, simply because no-one else had anything to trade (standard sized, archipelago map).
Building roads is far less important and as long as my cities link up, that's all that matters. Who cares if my miners have to lug ore across open country for 100 miles back to my city? Why would I want to build a road to another Empire to share and trade resources when I only need to open a trade window? Why would I want to build a road to my hill fort to stock units against the enemy, when one unit sat there is all I can possibly have sat there anyway?
CiV just feels very basic and rigid to me, like Catan. It's simplified things. If you open the strategic view it's practically a massive Catan map. The graphical look adds to this similarity in my opinion.
In previous versions of Civ I felt like I had a lot more direct control of my Empire and how it developed and grew. In CiV I feel like I can pretty much just go on auto pilot, occaisionally re-opening a trade agreement or restocking my limited army and connecting my newest city. I don't have to be involved anymore. What I build in my cities is almost irrelevant and I don't feel like I need to build anything desperately like I have in previous games. I haven't once needed a City Wall as a maximum priority or any of the later defensive buildings. I don't need harbour's because the bonus is next to useless unless I have an island city. I haven't felt the need to build an Arsenal, Military Base, Hydro Plant, Medial Lab, Hospital, Solar Plant, Stadium and rarely even a basic Colosseum.
I'm still playing it though and I haven't picked up another game after 4 days but I'm not sure how long this one will keep me interested. The achievements have been the one thing that I've been actually thinking about whilst playing. I need the achievements and if you think about that, it's pretty sad. The thing keeping me wanting to play is not CiV but the achievments.
I think the one overwhelming similarity though is once I played the first game, I don't think I've discovered anything different, exciting, unexpected or new in any of the subsequent games and that isn't Civ, that's a standard board game with rules and no surprises.