Mainly the wait. The paperwork and bureaucracy is fairly mild.
And I'll admit even that is dropping. I just got a correction letter today on a Form 1 to SBR my Wiselite Arms Sterling Sporter back to something that appears like an original Sterling.
Apparently my "S's" and my "5's" in my toddler scrawl are a little too close for comfort. I can't blame the NFA examiners, I had to go check the firearm myself to be sure.
I sent in the Form 1 on June 26th. Generally your approved stamp is coming in 2-3 weeks after the corrections are returned. Considering my first F1 to turn my Draco AK pistol into a SBR took 14 months from the beginning of April 2012, to the end of May 2013, that's a big change.
My cynical pessimist side just thinks they're "clearing the decks" for rule 41p next spring. My optimistic side wonders if the ATF would hire all that staff just to fire them in a year. (the assistants might be temps, but the new examiners are full time fed.gov employees...)