The big tests for Mongo are how well it behaves under heavy load, how easy it is to shard/replicate data at large scale (e.g. what Zawodny was talking about in his presentation about Craigslist having 100 MySQL boxes), and to what degree data is recoverable when inevitable failures happen (there are some big questions here with Mongo since it appears it trades off ACID compliance for speed). It's great to see bigger…
See http://mongodb-is-web-scale.com/ for a humorous angle on the speed vs. ACID compliance tradeoff.