Actually, humans have nine senses, I'll fill in the blanks:
Thermoception (temperature), nociception (Pain), proprioception (positions of various body parts in space),
Inception (the dream is real) and equilibrioception (balance).
The obvious choice here is "pain".
Kyse (who is sitting next to me as I type this) has pointed out that without a sense of pain, you would have no idea if something sliced open your chest and would likely bleed to death.
I responded with "I think I would notice if I was bleeding to death considering that bleeding to death entails blood gushing out of my body at an uncontrollable rate".
Kyse: Yes, but what if you were sleeping?
I countered this with "pain is different from pressure (see: sense of touch) the likely hood is that I would be awoken by the pressure from a butcher's knife slicing open my chest.. I'm a light sleeper"
Additionally, if I
must bleed to death from an open chest wound, I could calmly sit there and await my end, perhaps watching all the important moments of my life (season 3 of dr. who) roll past me or something.
And then maybe I'd contemplate my place in the universe and go "oh noes i didn't finish
de Profundis woe woe woe"
The upsides to not feeling pain are too good. I could get a sleeve of tattoos, a nose piercing, a facial cleanse, other weird practices that involve a paid professional sticking a needle in your body repeatedly without medical pretense.
Oh. But it would suck if I was a hemophiliac. Good thing I'm not...
The end :)