So, whats the secret to the design? What are the elements needed and how do you know if it looks Forrunner-ish?

1st you should study some pictures.
http://files.filefront.com/maniacs+angles+thingwmv/;9370982;/fileinfo.html
Only try to use 30 45 and 60 degree angles if u can.
Use slice plane a bunch (or cut tool) and dont make "pointy" stuff.

The forerunners are mysterious. They have random geometry and machines and stuff that appear to serve no purpose, are serve some perpose for halo that we will never know. But it looks cool and does something to affect game play (its real purpose lol). Forerunners always have more behind the geometry. Give the feeling that there is machinery behind and in the walls by using the "tech_dense" shader (I think) and other electronic type stuff. They also have holo control panels and lights. Like they said, look at some forerunner examples like the library an copy that style. Also, remember to add some forerunner sound effects and stuff.

wat the crap are u talking about?

read what hes saying and you might have an idea :|
hes talking about forerunner environments

That's the thing. they don't have buildings. more like giant random structures in the middle of no where that serve some purpose god only knows. We don't know what they were like or why they built the things they did the way they did. But their architecture is ancient, yet most of it is still functioning and active, and for the most part, scratchless. They use durable foreign materials and their structures and machinery is constructed in weird, elaborate, and seemingly unnecessary ways.

So basically what you end up with, are ancient mysterious structures that you can build to fit game play, and no one will care if they are practical or not, (unlike in human maps were you can't just stick a building in the middle of a street because it helps game play), but no one cares because they don't know enough about the forerunners to judge their constructions.

One other thing to remember is that outdoor maps need to look like the terrain was built over the structures, rather than the reverse.

If you have halo 3, look at cold storage and epitaph. Perfect examples.

and sandtrap and guardian and isolation and avalanche and construct and narrows and valhalla