Realistically, humans are pretty dang hard to kill, barring injury to a vital area (head, neck, major artery, etc). Think of all the COPS episodes you have seen where some jackass got shot, and is up walking around, wanting to beat the guy that shot him to hell. While D&D hp are cinematic, problems arise when the PCs get high level and can bully around scores of 1-2nd level characters.
My solution was to give all creatures bonus hp based on Con and size. A character at 1st level or its 1st hit die gets a number of extra hit points equal to its Con x its size modifier. Undead are considered to have a Con score of 20 for these purposes. Sizes are listed below:
Fine, Dimunitive: x1/10th
Tiny: x1/4
Small: x1/2
Medium: x1
Large: x2
Huge: x4
Gargantuan: x8
Colossal: x16
Yes, this does mean that a great wyrm red dragon with a 25 Con will have an extra 400 hp, and that the average kobold will have 7-8 hp.
Also, to borrow a convention from D20 Modern, I use Massive Damage Threshold (MDT) rules. Anytime a character takes more damage in a single hit than his (Con score + BAB) x size modifier from the table above, he must make a DC 10 Fort save +1 DC for every 3 points of damage over his MDT or fall to -1 hp immediately and start dying. For example: a 6th level human rogue with a 14 Con has a MDT of 18, while a 5th level ogre barbarian has a MDT of 40. This represents a lucky blow that causes great trauma and takes the character out. So far, these rules have worked really well for our group, being fair and without complicating combat with WP and VP. The players seem to like it, and it does make combat a little more realistic.