This is probably not the place to state Opinions about the database running on MongoDB, is it?
Lol, go ahead. Seems as good as a place as any. Though I expect you will struggle to find much of a person willing to defend the whole thing running on MongoDB. My sense is that opinions on the team range from "This is bad, but not bad enough to make it top priority to switch away from it" to "This is so bad that we immediately have to switch away from it".
We are running on it because the Open Source forum codebase I built the first prototypes for LessWrong 2.0 off was running on MongoDB, and that codebase had already built a lot of functionality I wanted for LW so it saved me a lot of work in the beginning, but I would almost certainly never choose it as a database if I had the choice to build something from scratch.
Multiple tech companies I've worked at have had "get off MongoDB" projects that took more than a year, so that colors my interpretation of it. It may be less bad if your team never grows beyond 10 people, I guess?
(That, and I've lost and mangled training data because of MongoDB.)
Migration is complete! Sorry again for the inconvenience, and I hope the downtime didn't end up being a major problem for anyone.
LessWrong's database provider (Mlab) has been acquired by a separate company (MongoDB) and as such our database needs to be migrated to that new provider. In order to do that, there will be about half an hour of downtime on Friday (April 10th) between 10PM and 11PM.
Sorry for the inconvenience.