I had posted this in the backer lounge, bur it seems like it's more appropriate here:
Hey guys, first time backer here and I'm glad my first Kickstarter had such as awesome community. Props to you guys (and Larian of course).
I'm really, really psyched for GM mode. Ever since I started DMing for my playgroup, I've wanted a system to automate things for me. That way I can spend more time crafting the story and lore, than speding brain power on minutiae like rules enforcement.
I can see why people think it would be difficult to DM online/in real time but I think it's comparable to tabletop RPGs. You would still spend a week beforehand crafting storylines and alternative scenarios for the characters, and during the game you just drop them into play and occasionally deal with curveballs the players throw at you. I agree with MechSoldier that narrative can be handled most easily through chat and voice.
In all, you would lose a bit of the interplay and socialization from tabletop gaming, but you gain a lot more in terms of rules automation, art assets, fewer chances for metagaming, and most important to me, the ability to play with people anywhere in the worlds.
A couple of things that I would like to see implemented in GM mode (let me know what you think) which would be really cool:
1) Procedural generation: You could generate everything from whole worlds to the layout and furnishings of a single room (I have the hardest time deciding what to put in dungeon rooms), maybe even NPCs, loot, and monsters
2) Session replay: Speaking from personal experience, my playgroup and I have a hard time remembering the details of things that happened a few sessions ago (especially when dealing with those curveballs I mentioned). It would be great to just be able to replay the pertinent parts for the group or an individual (less metagaming again)
Can't wait!

In general, I'm visualizing the GM mode as an online extension of the tabletop RPGs I know and love. 4 players and a DM seems ideal to me but the option to have more players/spectators is always welcome (50 is a bit much though). I'm not a big fan of letting the DM control monsters (unless it's like an endgame boss or special encounter).
In a tabletop RPG, monsters have rules to govern their behavior and to keep the game free from bias (one of the reasons dice rolling features heavily). These are systems that can easily be automated to free up the players and GM to focus on their main goals: The GM builds the world, and the players write the story.