Unfortunately you can't put in the Elephant (giant big moving base thingie) because it simply won't fit on most maps, I guess, but overall Forge is going to be awesome. I really wish it came with a fully-fledged map editor, too--I know it would clash with how tightly Bungie controls the Matchmaking service, but it would keep us from having to wait forever for new maps.

The coolest thing about Forge is that you can actually have up to 8 people playing a map as it's being modified. A common example is two teams, each with three Spartans and one Monitor, which is the form you take when editing: this would allow one person on each team to constantly supply their allies with weapons, vehicles, cover, or whatever, or allow them to smurf with the opponent's stuff by deleting their objects.

I'm sure I'll get the hang of it when I start using it, but right now Forge seems so good that I feel like I won't be able to adequately take advantage of it.