What.
That has GOT to be the single most impressive feature of Reach. The only thing that could compare with Forge world is...well...nothing, really. I can't believe that they even did that. 5 maps in one. That's...incredible. Simply incredible. The multitude of what you can do in this new forge is simply...unimaginable. You can't keep this in all in you're head at once, as Bungie says.
The tools are better, they have snaps for placing objects, and, coolest of all: The three modes for placing objects. These are Phased, Normal, and Fixed.
Normal is just as it is in Halo 3. When you drop something, its bounces and falls as it would in reality. That makes things harder when you are trying to place objects and the usual whatnot. Fixed mode is like Normal, except that when you let go of something, it stays where you put it. This is better for placing objects...in mid air. You can make islands with this. It's awesome.
The third mode is the best, and the best is Phased. Phased allows you to place objects inside of objects. This is great for making maps that are cliff side, or maps that are great for use with the new Jetpacks that you get as a SPARTAN super soldier. This game looks incredible overall.
I cannot wait.