I had the same problem and it turned out the computer was looking for an external hard drive to mount that wasn't connected properly. If you have any external hard drives, you may want to try unplugging them and then doing a forced restart?
My issue began after migrating from a time machine backup (10.9.2) so I can't imagine there's an OS conflict. I've done many migrations from time machine, and Mac to Mac (I'm no novice) and never experienced this issue. The external drive used for the backup isn't connected when I attempt to fast user switch, so at the moment I can't imagine what the system is stuck looking for since the exact data exist for all the users on my system. There are 3 users (A,B,C) on my system, and this issue only affect trying to fast user switch from 1 user to another (lets say A to B). The 3rd user C I can fast user switch to A or B without issue; so the issue lies between users A & B (or B to A). I can go from C to A or B though...weird.