Three warnings when using mongodb. None of these are enough to say not to use it, but they're things you need to watch out for:

1. Don't run javascript on a production db node. db.eval locks the node it's running on until it finishes, so the performance of that node will go down the tubes. Mapreduce is less bad in this regard because it does yield, but it does so too infrequently. If you want to use mongo's built-in javascript interpreter for anything other than development and administration, set up a slave to run your scripts on.

2. Don't use 1.6.1. If you're using 1.6.1 right now, upgrade to 1.6.2. 1.6.1 has a nasty crashing bug that had my mongo node going down about once a day and not coming up without running --repair.

3. Evaluate how much data loss costs you. Mongo stages writes in memory, and so if the db crashes hard it's likely that there will be some data that hasn't made it to disk yet. If you're building a social network the cost of some potential data loss is probably much less than the savings in hardare, admin costs, development costs, etc. But if you're a payment processor or a gambling site, stick with postgres.