I have a strange relationship with city-building games (Tropico games, Anno games, etc.) and Civilization games - I love them, I love playing them, but after I've played them a bit, I get rapidly bored.
I can only play a Civ game once a year. After I finish a playthrough, I can't bear to start another one. Doesn't matter which Civ game it is and that one playthrough is tons of fun, I just can't do another one. The only exception to this was Alpha Centauri (I at least consider it a part of the Civ series) due to the widely different playstyles of the various factions.
City building games are a favourite of mine, loved them since the early Patrician games and SimCity. However, I can go through the build-up period only so many times before I can't stomach it anymore. A recent example would be Tropico 4 - I'm 2/3 through the campaign, I fire up the next mission and then realize I'm about to build a bunch of mines, then a clinic, then a church, then a high school, then some farms, then get USSR development aid, then build apartments, then a weapons factory, etc. etc. And I just did the same thing in the last 12 missions. So I get a bit fed up and shut the game off. You might say "try a different build", but in city building games there pretty much is only one build - EVERYTHING. You need almost every building to get the best city with the happiest people, not much room for deviation. And so the tedium sets in...
EDIT: Oh, and just so I can piss people off: I almost fell asleep 1/3 of the way through Modern Warfare 2 campaign. The spastic ADD action was so uninteresting and unengaging that the game managed to make a russian invasion of the US seem tedious. Also, Oblivion was as dull as watching pain dry AND wathing grass grow, all together.