Sluggy.com: From MySQL to MongoDB
It took me a while to understand the real reasons behind the decision, but finally got it: object-relational impedance mismatch[1]:
Huge amounts of our code (as much as 80%) was dedicated to converting UI Objects to and from Database objects
Video and slides below (note the above hint is on slide 18 and at around minute 10 in the video)
This post is part of the MongoDB Case Studies series.