Actually - two of them. They are:
1. If the GM assigns a character to someone else - that someone can not do anything with them because no new radio button shows up at the bottom of the screen.
2. The notes for a given character can not be seen by the player. This makes it kind of hard to: 1)Pass notes to someone in this manner so they don't show up on the chat screen, and 2)The player can not view these notes. So if a character is losing items because a thief has stolen something or ONLY that particular character sees/hears/whatever something and you only want them to know about it - it is not possible to do so without alerting everyone else. This should be a two-way deal where if the player puts something into their notes - the GM can pick it up and if the GM puts something into their notes - the player can pick it up.
Of the two, #1
has the greatest impact because once a GM takes control of someone's character - they are pretty much hosed and have to get off - get back on again.
Easiest way to test this is to have a GM and player get on. Have the GM make a character and then try to assign it to the player. Won't do it for me. :-(