Hurrah - good to see that the Elasticsearch dependency is no longer locked to a rather old version (that bit me a few times when setting up a cluster last year). I'm still a bit wary about having to spin up MongoDB for it as well, though…
The MongoDB requirement gives me pause, too. I've wanted to give ToroDB (https://github.com/torodb/torodb) a try for awhile now, and I think I'm going to see if Graylog will talk to it for grins.