Why is it so nasty? Well, its the most electronegative element, so it will rip off whatever other element is stuck to a metal and replace it with its own. It'll eat through concrete or glass, spontaneously combust with chocolate, and is very toxic to most life.
I had a chance to work with HF (hydrofluoric acid) in APh 9, a freshman integrated circuit lab class. Its used for etching the chips, so we were all going to be working with it. We got quite a stern safety lecture, because despite its dangerous properties, HF dissolved in water (even in moderate concentrations) doesn't seem dangerous. See, because its already stuck to a hydrogen atom, it doesn't let go very fast and so it reacts very slowly and quietly. No dramatic bubbling or toxic fumes, just a slow silent etching of the exposed Si to create the chip features.
So what invariably happens, is that someone can't get their chip out of the little plastic cup with the tweezers that we're supposed to use. Instead, they look at the chip sitting there in the bottom and think, "hey, I can just reach in there and get it out." When they do that, nothing happens immediately, but a chain of events has started that can be deadly.
HF acid won't burn your skin even. Instead, the F ions start digging down into your tissue, poisoning enzymes and other doing other havoc, till it gets to your bones and starts binding with the calcium there. This continues for the next 24 hours until you loose a limb, or worse, die. It doesn't really take much as these safety reports show:
A dermal exposure hydrofluoric acid over a 2.5 percent total body surface area resulted in death. The serum calcium level was 2.2 milligrams/deciliter.
A patient with HF burns died from intractable cardiac arrhythmia secondary to the depletion of ionized calcium.
Estimates of the lowest lethal concentrations for hydrogen fluoride range from 50-250 ppm for 5-minute exposure and are based on accidental, voluntary and occupational exposure information.
Luckily, in our lab, another student or the TA usually noticed this act of stupidity, and would call 911 while having someone rinse off the area affected. The "stupid frosh" would then be whisked to the hospital for burn treatment and calcium injections (in an attempt to bind all the loose F before it got to the bones and to prevent heart problems). As of 1985 when I took the class, there had not been any fatalities yet.
In any case, I'll agree with the other Derek. Fluorine is not something I have any desire to mess with again.
Derek Lowe's, Things I won't touch (part 1)