Two things:
MTC out. I love that Live can receive MTC but I use Live on stage and want to trigger a Digi Venue and Light board to have mixing and lighting automation synced with the music. Right now I have to rewire a 3rd party app to send MTC and it doesnt always play nice.
When using 2 sessions of ableton on 2 computers i always sync them up. works great except for 1 thing. if im playing a soft synth on the slave machine, and i hit transport stop on the master machine. the audio output of the slave stops immediately. now i understand why this would be wanted/needed. but it would be amazing if there was a toggle to turn this on and off.