I still have build 18, which I believe is the last build before they started charging. I have some mods, but I accidentally deleted most of them. The one I still have and love, though, is Invasion(I think). You have this little bunker in the middle of a big field, with giant hedgehog tank traps'n'such. It's best if you don't put any fortifications down that aren't already on the map, and just give the leader a flak cannon. After about an hour, the map is this hideous mass-grave, littered with downed dropships, some of which are still running, creating lethal obstacles(Stepping on a still-running dropship causes a physics glitch which tears off your legs, and if the thrusters are shooting off into the air, flying through the emission will vaporize you), and the dropships are glued together with this green-and-red moldy looking substance, which you realize with horror is the bodies of hundreds upon hundreds of dead soldiers, smashed to paste by the falling dropships.
If you get bored of fighting, you can do another fun thing. Load a dropship up with soldiers, and fly sideways, just barely pressing the thrusters, until you're going at ridiculous speeds. Go until you hit an enemy dropship, and as you're spinning through the air, spewing fire and sparks, open the bay doors. Watch your soldiers fly across the battlefield at obscene speeds, so fast that when they hit the ground they're pulverized into red mist. Even more awesome is when they hit a dropship on the way down. By that time you'll be going so fast that you'll fly through the blood as you z-wrap around and pass the spot where they hit. Sometimes they even survive, and it can be cool to play as the sole survivor, wounded and surrounded by enemy soldiers.
Also, yes, the physics are great. When soldiers walk across soft ground, they leave little footprints. When Space Marine Dreadnoughts(Courtesy of a mod) walk across the ground, they leave large footprints. It's too bad that the dreadnoughts aren't that good. They're very resilient, and no amount of regular bullets can even faze them, but their joints are very fragile. If they get weighed down in sand, the force needed to get out can cause the legs to come off.
EDIT:
Somehow I forgot to point out, always play on the hardest difficulty. The guys can't get to your brain anyway, unless you get the Crazy Burrowing Dropship Glitch(Something happens occasionally that causes a dropship or rocket to move so fast it becomes a matter annihilator, burrowing a huge cave through the ground and lagging the game like crazy) and it hits the brain.
EDIT2:
OH! I almost forgot, I once had a falling dropship land on a soldier, and he survived. It was insane, I was sure he'd be goo, but he kept himself together. By the time he died(About three seconds later), he had bled so much the blood had run all the way across the top of the bunker and was pooling on the little table above the brain canister(You'd know what I meant if you saw it, I might upload the mod to something, since it's probably dead now that that build is obsolete.)
tl;dr
Read it, lazy. I'm not condensing it for you.