1) slugs - Dude, anyone who disagrees with this as number one has obviously never stepped on a slug, and lucky for them. I can't imagine anything more wholly unpleasant to step on.
2) Legos - I own friends (yes, I own my friends. Well, not all of them, but some) who swear that no more painful experience exists then to step upon a Lego barefoot
3) spikes - ya'll think I'm being silly, but I'm not. We have this tree in my front yard, I hate it, it doesn't grow anything useful like leaves or fruit. Instead it's just covered in three-inch-long spikes. That sounds cool, but it really sucks to have one sticking out of your bare foot. It also sucks to mow around.
4) broken glass/hot coals - broken glass sounds bad but I used to walk barefoot in the alley outside my house all the time and got to where broken glass didn't bother my feet at all. I've stepped on stray hot coals before and it sucks but then some people walk across beds of them for fun so they can't be too bad, right?
5) banana peels - nah, I'm just playin. Banana peels aren't actually all that slippery.
Man, I can't believe the second most recent entry here (not counting this one) was me almost a year ago. Heh, Matt stops using LiveJournal, and everything goes to heck. Or to Hell, even. Some terribly unpleasant place, I'm sure. Purgatory?
top five unpleasant things to step on whilst barefoot: