Don't need transactions?
They can believe whatever they want, but in the real world, transactional integrity is very important. Even more so when you're talking about clusters where the user's session can bounce like a ping pong ball between servers.
At least pragmatism prevailed and they pulled their head out of their backsides long enough to address the single biggest criticism people had about Mongo.
Now if only they came up with a consisted language for operating on NoSQL databases. Something structured.... Yeah... like... a structured query language... Hmm....