I was active in the previous astronomy SE, and my impression is that it failed because of lack of questions. I think what questions we had were of reasonable quality, not really distinguishable from the ones I have seen here, and they usually got good answers, but as time went on there just weren't enough of them.
My overall impression of the nature of the community was that there were a small number of enthusiastic members, some of whom were quite knowledgeable. So, questions got answered (and answered well), but once this core community ran out ideas for seed questions, there were too few questions from people outside of this core to sustain the site.
So, I think the challenge this community needs to address to avoid the same fate is how to attract more questions, hopefully without sacrificing quality. This subjective impression matches up pretty well with what I see on the final stats for the previous beta.
On the meta side, we had many of the same questions about what is "on topic" as I see on this meta (theoretical questions allowed? amateur only, professional only, or both? What about questions on astronomy software?)