So I did try mathoverflow a bit and it is a cool site. Over the few days I spent there I gained 593 reputation points, and no less than 9 bronze badges. The first answer I proposed gave me a badge as “teacher”, and the first question I asked gave me a badge as “student”. In fact, if I will only reveil my age I would gain also a badge as “autobiographer”.
Indeed, mathoverflow is ran by an energetic and impressive group of very (very very) young people (more precisely, very very young men). Here is the link to the 35 users with highest reputations.
Update: Well, I got a little addicted and visited quite a bit this nice site. Among other things, I tried to promote my fundamental examples question. Basic examples can give a quick invitation to wide areas of mathematics. (Maybe to most areas, I find it hard to think about a counterexample.) Overall, there are not that many basic examples and I think there will be a consensus about most of them so lists of different mathematicians will not be so different. More examples of important examples are welcome.
In my profile you can find there the six questions that I asked and the 10 answers or so that I offered (to other questions).
There is no definite answer yet to my question about typical behavior of random noise. (Update: Greg Kuberberg gave a convincing answer.) I still hope for an answer here or there. But I did found a new home to (and additional information on) functions of intermediate growth, like the function that satisfies the relation 
If you like discrete geometry you may be interested in this speculative question related to the Gallai-Sylvester theorem. (Update: some answers have led to a formulation of clear open problems and to some useful insight.) Overall, there are not many items in mathoverflow so far related to discrete/concrete mathematics. If you wonder what categorification is, look here for a few answers.
If you have an example of a mathematical example that has shaped a whole area of mathematics you can contribute it here. And if you want to tell a mathematical joke, or read quite a few, you can look at the question “do good math joke exist?”
Combinatorics and more 
Now that you point it out, it’s striking to me how male the regular users of the site
are; the first user who’s name looks female to me (some users have gender neutral names, so one can’t be sure) is about 100th in reputation, and only reached that point in the last couple of days. This is very striking, even in mathematics (after all, the ratio of males to females amongst graduate students and recent Ph.D.s in the US is under 2). Hmm, we may need to think about this point…
Hi Ben,
I do not think it has much to do with math. Look, for example, at the programming analog, and again the main users are male
http://stackoverflow.com/users
and it is really not clear what drive people to be so active: e.g. to answers many quesations every days for over a year.
This “hyperactivity” seems quite unuseful for the individuals involved as well as for the whole endeavor.
There may also be an effect of “old boys network” (or young boys in this case) in the evaluation process but this is a second order effect.
To be honest your random noise question still confuses me; I’m not sure what you’re fishing for other than a non-homogeneous Poisson process.
Dear Jason, I suppose it is a bit confusing. Maybe what I want to ask is simply: What is the rihgt notion of a “random probability distribution” on {0,1}^n or on {0,1,2,…,r}^n.
I would probably add the following informal non-comitting rules of conduct for mathoverflow:
1) over time do not ask more than 1-2 questions a week.
2) If you are a prolific answerer (you provide more than 3-4 answers a week) except for a few questions that interest you most, wait 1-2 days before you propose an answer to a question.
I like these rules of conduct! I think I agree that the hyperactivity degrades the average quality of the questions and answers somewhat. But for the moment, maybe this is due to the “novelty” of the site (people are having fun playing around with it at the moment), and after a while people’s interaction will be a bit more sober.
I think for the moment it’s still important to define MO’s scope, and for that reason I, for one, would like to see slightly more questions asked – but then again, I am one of the most flagrant violators of both of those rules!
Dear Danny and Qiaochu,
Right I am not sure at all that these proposed “rules” are good and, in any case, I propose them for the “stationary” MO state. As I said, the MO looks very successfull and useful. The answers and questions given by the most active members, Ben, David, Qiochu, Reid, Anton, IlyaN, Scott, Eric, Tyler, David, Greg, Kevin Andrew… are impressive.
In the stationary state it will be good if there will be a large circle of people answering the more mundane questions, and perhaps some experts answering very pointed questions. Also it will be good if the scope of questions will become larger. (There are some incentives now to give answers very quickly, within minutes perhaps, and this is a little unusual.)
I have just looked at Stack Overflow and one person there has reached over a hundred thousand points through over 6000 answers over one year.
So apparently some people can keep up the pace over a long period time. So the same thing might happen in MathOverflow.
I think I’ve settled into following both of those rules at least for myself (although I think “soft” questions are probably fair game for answering more often). But I feel oddly guilty about not asking more questions, and about not having more questions to ask! I also think I agree with Qiaochu, in that it’s probably more important to figure out just what MO is right now than to be concerned about there being too many questions.