Of the twenty five obstacles at Tough Mudder, four really stood out to me. Hero Wall set the tone for the day and is probably what I associate most with Tough Mudder now, but that's been covered already.
Block Ness Monster. The first water obstacle. Water that comes up to your shoulder and two large rotating blocks to clamber over in the middle. I think that jumping into the water while fully dressed was such a disconnect from the norm that I ignored the British convention of moderating your voice in public to start barking encouragement to people to get the block rotating.
Cage Crawl. A long channel of water with chicken wire draped an inch over the top. Maybe it was the cold, maybe it was the sense of being trapped, but on this obstacle I found myself breathing quite heavily, and with that I sucked down a mouthful of filthy brown water. It wasn't until the next day that the sensation of it hitting the back of my throat disappeared. Pretty gnarly really.
Pyramid Scheme. Just a tall, inclined sheet of plastic that you need to climb up. It's surprising how much weight you can support when you're helping people up a slope. There was this one guy, must have weighed twice as much as me, and getting them up the slope wasn't a challenge. Admittedly, if people weren't careful they could easily kick you in the head, but I only saw that happen once, and quite importantly not to me.
Electroshock Therapy. A generator from an electric fence hooked up to a spread out beaded curtain. I got shocked quite a few times on my way through and each time I could “see” my calves light up. It was like a translucent overlay on an image. Wild.