"This (mongodb's write plolicy) is kind of like using UDP for data that you care about getting somewhere, it’s theoretically faster but the importance of making sure your data gets somewhere and is accessible is almost always more important."
"It's okay to accept trade-offs with whatever database you choose to your own liking. However, in my opinion, the potential of losing all your data when you use kill -9 to stop it should not be one of them, nor should accepting that you always need a slave to achieve any level of durability. The problem is less with the fact that it's MongoDB's current way of doing persistence, it's with people implying that it's a seemingly good choice. I don't accept it as such."
But if what you say is generally true, then MongoDB is giving a terrible showing for software that's past beta.