Yeah, it's worse than it sounds though. If you write to 2/3 nodes, and the third gets elected, you can roll back majority confirmed writes. I think it did even worse things when near capacity, but those are harder to pin down.
So the "majority" write concern is not safe. However it seems that the number of nodes that must confirm a write can be configured http://docs.mongodb.org/manual/core/replica-set-write-concer...
From what I read there it seems that if you want to get a confirmation from all the nodes and one gets offline you'll be waiting forever (but there is wtimeout). I'm afraid that any db cluster with any technology has similar problems.