It's not so much that Far Cry 2 was a terrible game, it's not. It had a good core to it but the flaws were especially glaring given the annoyance they present to the player. The big deal is that Far Cry 2 just isn't Far Cry. Far Cry 1 felt like a totally different universe compared to the sequel and the gameplay elements were a lot different.
Far Cry 2 would have been better off as just an original IP but of course they wanted to do something useful with the name.