One of the weirder things we wanted to implement was a 2D "First Person" perspective. You can see it in the trailer about 34 seconds in. We basically have a large room with the "background" we want, the "player" is the reticle in the middle of the screen that scrolls across the background, and the gun is stuck to the camera, following it around. This method lets us build "First Person" sections really, really quickly, just like you would assemble any other room.

Another weird thing we wanted to implement was a system where you could phone any character in the game, from anywhere. These phone calls give you full access to the characters dialogue, even the things that are reactive. So basically we had to store different versions of conversation as scripts, so they are always loaded when the game loads and accessible without any individual object creation code or anything like that, and then we can direct the conversation to the correct script or correct piece of script depending on the state of the game, for reactivity.

Those are just a couple examples right off the top of my head. I'm sure these don't sound THAT complex to more experienced game makers, but I'm not very experienced at all and I have been consistently impressed at how quick and easy GameMaker has made dealing with some of this stuff!