Done it three times (last 3 years), and doing 2013 too. Only ever in normal running shorts and vest. I can't therefore comment on other suggestions, but nothing has made me want to try any other type of clothing because TBH the cold only kicks in as a serious problem *after* you're soaking wet, so anything thick and heavy will do virtually nothing useful prior to the water and be an absolute nightmare afterwards. I have nothing but respect for the lunatics who try it in fancy dress, but it must be horrific (the big stuff that is - those who run virtually naked are probably no worse off than anyone else). Obviously neoprene would be a different matter, but then I kind of think that's not the Tough Guy 'spirit' - the event isn't really intended to be something a bit like a triathalon - it's all *about* the extreme conditions, rather than trying to avoid them.
The biggest mistake I made the first time (and never again!) was to not arrange quick access to a towel and lots of thick dry clothes after the race. My bag was with a friend and I was semi-hypothermic trying to find them for about 15 minutes - the worst part of the day by far! Pack a bag with lots of warm clothes and make an accurate mental note of exactly where you left it, rather than leaving it with a friend - it's so busy and maze-like around the finish you might not find each other quickly.
Fingerless gloves are essential for the ropes, but you can buy these on the day.
Edited: 15/10/2012 at 07:07