The Retro-Roleplaying blog has a post on whether complexity really discourages casual gamers or not. That's not a question I want to deal with directly right now, but I wanted to point out that when we argue about game complexity, there's at least two kinds of complexity, which may account for the lack of agreement.
Let's humorously refer to these two kinds of complexity as tyranny and incest*. Tyranny is the degree of control that the rules have over detail; you can judge how tyrannical a system is by the number of steps involved to get the answer to a question, like "who wins the combat?" If every round of combat requires an initiative roll, an attack role, a defense roll, a shield bonus roll, a weapon save roll (for breakage,) a damage roll, a damage resistance roll, a system shock roll, and a morale roll, that's an extremely tyrannical combat routine. Depending on the desires of the players and ameliorating provisions, like a unified mechanic for all roll types, simultaneous rolls, or the opportunity to skip rolls, it might be an acceptable level of tyranny.
Rules incest, on the other hand, is complexity between subsystems -- one thing having an unexpected side effect on another. Again, if there are provisions that make a highly incestuous rules system easier to handle, it might be a bearable form of incest.
The main problems with tyranny are remembering all the steps and spending the time to follow them all. A cheat sheet and a handful of reusable techniques can fix the first part, but in general, the more tyrannical the rules, the more time it takes to resolve issues. This is a linear, additive increase in time.
The main problems with incest are remembering all the ways subsystems incestuously interact with each other and dramatic increase in time figuring out the incestuous effects. Unlike tyranny, the time needed to resolve questions in an incestuous system is based on permutation rather than addition: it's a factorial.
Maybe I'll talk about which systems I consider tyrannical and which I consider incestuous in a future post.
* And to make sure that all systems are equally insulted, the inverse of a tyrannical system is a hippie-communist system, while the inverse of an incestuous system is a wimpy-simplistic system.