I think the most important thing to know is the new LOM and what changes have been made to Max for Live in details.
As far as I know nothing has been published yet. It would be great to get some details. Current crossgrade sale is interesting but only makes sense to me if the LOM is expanded and shortcomings of the current API have been solved. On top of my wishlist is creation of clips in Ableton and proper functions to create cue points in clips... Stil dream of a Max for Live powered Browser to hold my DJ Collection and being able to load Files from m4l Device in a Live Clip.
My personal wish for M4L API is that standard max UI objects ( jsui included ) can be seen by the Midi/Key learn system in Ableton Live. I'd like to give my devices a personal GUI. And i'm quite tired of live.x UI objects appearence :-P
my favourite is the convolution reverb. The work Alex Harker and the Ableton designing team have done around the HISStools is just mind-blowing. It sounds amazing and they go some creative places that I would have not dreamed of... nice one!