For better or worse, most development discussion happens on GitHub rather than julia-dev. I've been catching up on GitHub threads recently, and the progress towards 0.5 is really impressive -- both in the language itself, and in the tooling (especially the debugger). I think the nature of priorities has changed somewhat as the language matures, but I disagree that development has slowed. (disclosure: I'm a Julia committer, but have been a bit out of the loop for some months now)

There is certainly a need for better interim communication on progress between releases, to let people know what is happening without reading every GitHub thread.

No summer of code students have had projects that involved work primarily on the core language since 2013. Not that many posts happen on julia-dev because most of the development communication happens on github.

I do think it's seasonal, however not connected to GSoC. Many of the major contributors are (or were) students (Julia seems to be detrimental to people finishing their PhDs, except in the case of Jeff Bezanson, where in some sense he crowdsourced a lot of the research, as Julia was the topic of his thesis ;-) ) Many who aren't students are professors. Definite drop off when people are taking classes, exams, teaching, etc.