Joel and Jeff have stated that the reputation system is key to stimulating the kind of user behavior they want to see at StackOverflow and friends. Well, it's not working particularly well for me right now. After two and a half months, my rep has plateaued at 124, and I'm no longer interested in spending a lot of effort to get it much higher. I'm not looking for a pity party here, but I suspect there are other users in the same boat, and if the reputation system is failing to motivate us, that might be a topic of general concern to the management.
I think a major part of the problem is that the topics I'm writing on (C, C++, and algorithms) don't get as much attention from the community here as other topics, and that makes it hard to earn reputation points by spending time answering them. Of the 16 answers I've written, four of them have been accepted as the official answer by the author (which accounts for most of my rep points), but none have more than two votes, and only five answers have any upvotes at all. Basically, it looks like if the author really likes my answer, I'll get an upvote from him/her and maybe one other person, but otherwise my efforts just go "plonk." In most cases, it looks like if I don't answer a question when it first comes out, nobody is going to upvote it later, which limits the rewards for going back through the archives looking for questions I can contribute to, Here's an example: in response to a question about what the ternary operator is needed for in C/C++, other people mentioned the fact that it is an expression, not a statement, but I was the only one to tie that to the use in function-like macros and give an example where you needed a function-like macro in C because a function couldn't give you the necessary type polymorphism. An hour or so writing that answer, but I answered on May 1, and the only answers getting upvotes were written April 17, the day the question came out. Plonk. (Yeah, I may get an upvote or two from mentioning it here, but that's not my main point, which is that providing a unique perspective to questions that are a couple of weeks old doesn't seem to be a profitable use of my time, in general).
So - it's nice to be above 100 points, so that I can downvote something if necessary, and it was really frustrating before I got up to 50 points so I could comment, but most of the other goals that seem worth shooting for are so far out of reach at this rate that it doesn't seem worth the effort. Given a choice between spending time on the job I'm getting paid for, or constantly refreshing StackOverflow in the hopes I can pick off a relevant question a few seconds after it gets asked, I know which one offers the better reward-to-effort ratio. I don't really have a lot of questions that could be asked here without a fair amount of effort to strip them of proprietary information, so that doesn't seem like a good way to points either (the Meta site here might be different in that respect).
I'm sure my experience would be different if I were writing about PHP, or C#, or something more popular like that. But if you really want StackOverflow to be the font of all programming expertise, there needs to be a better way to reward those who answer older questions on rarer topics. Because right now, to me, it looks like that is mostly an exercise in frustration.