The question:

5$\begingroup$ The complementary question would be: "What areas/aspects of mathematics are overrepresented on MO?". $\endgroup$ – Stefan Kohl Aug 31 '15 at 20:22

12$\begingroup$ Dear Stefan, I dont think this is a good "complementary question," and, in any case, I did not ask it. $\endgroup$ – Gil Kalai Aug 31 '15 at 20:29

4$\begingroup$ I agree that to list things that are overrepresented is asking for trouble. Let us hope that on your question users will be more respectful regarding what you did not ask than we saw lately elsewhere. Other than that the data comparing arXiv and MO is a starting point. $\endgroup$ – user9072 Aug 31 '15 at 20:52

1$\begingroup$ Dear quid, Since we do not have a limited capacity, I don't even understand the notion of "overrepresentation" in MOcontext. Larger scope and better covering of underrepresented areas, may well be beneficial for those interested in wellrepresented area. $\endgroup$ – Gil Kalai Aug 31 '15 at 21:44

1$\begingroup$ I should have written "perceived as overrepresented" since my point was just to agree with you that it is better not to entertain the question what if anything might be overrepresented (as opposed to trying to hint at the existence of overrepresented areas/aspects). However, abstractly, I do not think that your argument that as we do not have a limited capacity this is entirely a nonissue is sound. I think would all of a sudden 300 combinatorics question appear each day, then you and I might like this but maybe some would find it harder to find the content they are interested in. $\endgroup$ – user9072 Aug 31 '15 at 22:22

$\begingroup$ We are in agreement, quid! When I talked about "MOcontext," I referred to the MO reality, and not abstractly. $\endgroup$ – Gil Kalai Sep 1 '15 at 7:46

$\begingroup$ related question meta.mathoverflow.net/q/2363/454 we see Algebraic Geometry and Number Theory are above, while Mathematical Physics and Information Theory are below. $\endgroup$ – Gerald Edgar Sep 2 '15 at 15:16

2$\begingroup$ Possibly related? ams.org/notices/201011/rtx101101421p.pdf $\endgroup$ – Timothy Chow Sep 4 '15 at 16:11

$\begingroup$ @TimothyChow  a causual glance suggests to me that the biases of PAMS, TAMS, and MO are significantly correlated. I would like to know what the numbers for SIAM journals do in comparison and in superposition. $\endgroup$ – Steve Huntsman Sep 4 '15 at 19:41

5$\begingroup$ Can all those not present raise your hands? $\endgroup$ – Chris Ramsey Sep 8 '15 at 14:34
IMO applied mathematics is underrepresented across disciplines.

7$\begingroup$ Where is your standard link this time? :) More seriously, I agree with the observation. It might be hard to change though as some of the potential traffic is likely already firmly directed to other (SE) sites. Still I feel (or rather felt, I pay less attention lately) the site is sometimes unwelcoming to more applied questions, which is something that one might try to fix. $\endgroup$ – user9072 Sep 1 '15 at 10:31

$\begingroup$ Underrepresentation of applied mathematics was mentioned a few times back in the "tea" days, e.g., in this thread: tea.mathoverflow.net/discussion/1483/1/… $\endgroup$ – Timothy Chow Sep 4 '15 at 16:13

$\begingroup$ @TimothyChow: There's my standard link! $\endgroup$ – Steve Huntsman Sep 4 '15 at 19:28

$\begingroup$ The link in the above comment does not work at the moment, but it can be found in the Wayback Machine or on my copy. $\endgroup$ – Martin Sleziak Jan 15 at 5:56
Ergodic theory usually has several papers in Annals and Inventiones each year, and is regularly mentioned when the work of recent Fields medalists is described (e.g. Avila, Lindenstrauss, Mirzakhani) but I think that there are no more than ten ergodic theorists who contribute regularly to MathOverflow.
Here is a nice statistic: since MathOverflow opened, there have been as many Fields Medals awarded to researchers active in ergodic theory as there have been bronze tag badges issued for answering ergodic theory questions on MathOverflow. Based on current rates of progress, the first silver tag badge for ergodic theory will be awarded to Anthony Quas at some point in the spring of 2022.

5$\begingroup$ The fact that a nonspecialist (yours truly) formed the "ergodictheory" tag many months after MO started up is also pretty solid evidence that the field was underrepresented from the beginning. But I am very glad that it had some knowledgeable representatives who could answer my elementary questions! $\endgroup$ – Steve Huntsman Sep 8 '15 at 1:23

1$\begingroup$ One question I am curious about is what is the origin of the name "ergodic" what does it mean and what is its history. $\endgroup$ – Gil Kalai Sep 8 '15 at 16:27

1$\begingroup$ @GilKalai: see note 8 of plato.stanford.edu/entries/statphysBoltzmann/notes.html $\endgroup$ – Steve Huntsman Sep 8 '15 at 20:22

2$\begingroup$ I would like to add that the cousins of Ergodic theory, i.e. various flavors of dynamical systems are also quite under represented here. $\endgroup$ – Piyush Grover Sep 9 '15 at 6:46

$\begingroup$ For the sake of saving the trip to a rather interesting footnote, the conclusion of the reference proposed by @SteveHuntsman is that the term comes from the Ancient Greek έργο (work) and ὁδός (path, road, way). $\endgroup$ – François G. Dorais♦ Sep 10 '15 at 23:39
With Keith Kearnes participating on MathOverflow (and hopefully asking questions soon), I am confident that Universal Algebra will be represented to my satisfaction. Taking that personal perspective as a benchmark, I turn to the Tags page and note that the universalalgebra tag has been used 20 times this year for questions. I suggest that any tag that a) represents a sizeable area of mathematics, and b) has fewer than 20 questions asked this year is a good candidate for an underrepresented area. In particular, conformalgeometry, mathematicalwriting, and combinatorialgametheory are underrepresented by this measure. I know I would like to see more questions in the last two areas.
Gerhard "And More Pictures From Joseph" Paseman, 2015.08.31
I would like to see more questions in freeprobability, whose tag has 13 questions, or more generally in noncommutative probability. While this may be a relatively small area of mathematics it should interest many parties, namely, probability, combinatorics, and functional analysis.