Bruno makes a few good points, to which I will add another one.
Rep is generated by voting. As of this writing, there have been three huge waves of votes:
- Private Beta. I haven't participated in it, so I have to speculate to some degree, but it seems reasonable to me to believe that the first few users voted rather heavily to sort things out, to set the general tone of the site.
- The first few weeks of public beta. A wave of new users came in, including myself. If memory serves me correctly, when I first registered on this site, there were already 400-odd questions and 800-odd answers. Many sounded interesting, so I had to "catch up" with voting. Since there's a limit of 30 votes/day, that phase was rather long. I was reliably handing out hundreds of rep points for many days in a row, even on weekends, when barely any new questions were posted. The same must be true for a rather significant number of other users. Now that we have caught up, things are back to normal. I am not handing out anywhere as much rep nowadays, and probably close to none on Sundays. After all, I can't vote on things twice.
- Somewhere in between, there were two questions that got promoted by Joel and Jeff, resulting in yet another spike in voting activity. Most of those users were passers-by; they left a few votes on those particular questions, but didn't become regulars.
I guess all three points can be summed up as follows: Things are at their normal level now. Rather than the activity being exceptionally low right now, it used to be exceptionally high in the past. Until the next big wave of votes — for whatever reasons it might come — it's business as usual.
Oh, and let me not forget that recently, our most active voter so far, kiamlaluno (800+ votes), went missing in action for about two weeks.
Lastly, to address your question about a general pattern, there are two theories that are quite popular on the meta Meta, even though evidence for both is rather anecdotal:
- As a site matures and certain users become "recognized experts" — at least as far as their rep is concerned — people tend to give more weight to their answers. In other words, high-rep users pile up even more points (comparatively) than low-rep ones. So, after some time, it's not at all unusual to see the same handful of users fighting for the top position in the reputation leagues, followed by lots and lots of other users who are not gaining anywhere as much rep.
- The other theory is that as time passes, regular users tend to cast fewer votes on average. Robert Cartaino calls it "The 'Everything Is in Order Here' Syndrome".
There are quite a few questions on the meta Meta that deal with these theories. I will link to just one, because it actually covers both of them: "Does herd behavior influence the SO sites?".