delericho
Legend
While there are multiple moving parts, so to speak, they are each of a rather simple nature and normally seem fairly intuitive in application.
It's not the individual moving parts; it's the interactions! Especially since, until quite late on (PHB2), 3e* didn't by default allow retraining, that meant that at each step you had to be sure to pick up the right thing, or you were at your DM's mercy to swap it out. In the worst-case, you might have to have your 20-level build worked out from the outset of the campaign, so that you didn't get hit with those nasty opportunity costs... or, of course, not worry about it, and just have fun with your less-than-perfectly-optimal character... but that's crazy-talk.

* Not sure about Pathfinder and retraining, but it inherited a lot, including a lot of assumptions, from 3e.