I submitted my resume online and got a response within a few days, asking for an initial interview. I said yes, of course, and had a half hour interview with one of the technical recruiters. She asked about my resume and MongoDB - likes and dislikes, what database experience I had, etc. There weren't any technical questions, per se, it was more to get a sense of my background.
I got the next interview, another phone interview, which lasted about an hour. This time it was a technical interview. It was my first technical interview, so I stumbled through it and didn't make it to the next round. I've done multiple interviews since and now realize that Mongo actually asks some of the easier questions, when compared to their peer companies. With that said, if you're excited about working here like I once was, make sure it's not the first company you interview with!