First and foremost, with the GM Mode, whatever we are allowed to do, it has to be stable. I know Jeff was saying this very thing in that they will have to look at what can be done on the fly safely to keep from crashing the game and ruining people's experience. Traps, enemies, etc... should all be easy to place if they are already created and available as assets to the game. Creating them on the fly while people are in game? Mmm yeah no as you need to test the pieces before letting them hit. Granted, slight modifications to stats wouldn't be too bad.
Effectively I see the setup like this, GM Mode is there to help you bring a more active life to your mod/story. It's how to can react, to an extent, in response to the players. GM Mode is not for big sweeping changes because the players did something you did not expect. If all you want to do is play the random antagonist to the players, this will be great by itself. Especially if you can take control of the mobs themselves. Effectively I see this as a tool to interact with what's already there and spawn in new items/creatures/traps that were either part of the game, or created by you (or someone else) in the mod tools. For dialogue.. mm.. that one is tricky. Setting up dialogue boxes and choices on the fly is not the best really... but I could see allowing GMs and players to 'say' things and it appear in a bubble or even just the info window. This would allow easy on the fly conversations between NPCs and PCs.
However, the Divinity Engine (Mod Tools) is where we build our new areas and assemble our own stories to tell. We lay our foundations and planned encounters here as well as anything that we need customized. Yes it will take time and be some rather involved work. Here I'd really like to see us be given a huge berth on what's possible to manipulate about the game. Assuming the game's logic, skills, and mechanics and such are all using a scripting language this should be fairly simple to allow... basically a "edit at your own risk" type of thing since you'll need to have a good understanding to make such deep mods. While work is going to be done to make this user friendly and easier to use by non-devs, it will still remain that it will take dedicated people to push what can be done to its limits.
I'm fully excited about to possibilities coming by using both tools. However, I will admit, I'd rather see GM Mode not be released with the game as both the enhanced mod tools and GM Mode are going to take a lot of work. I'd much rather see their release come after the game itself rather than delaying the release of the game until the tools were ready as well.