Many systems already account in part for this via spells and spell-like abilities that allow characters to "resurrect" fallen comrades. Including a Church with NPCs that can revive a dead PC is always a viable solution. Traveling to Hell or fighting Heaven to retrieve the soul can be an elegant way to add more depth to any campaign and extend possible encounters. I think most GM's tend to overlook this and "nerf" encounters for fear of derailing the adventure.
We occasionally use a house rule that allows a player to revive post combat death with the loss of a limb. I am not a huge fan of this in all cases, as it could prove far more detrimental to the character making them even less effective and more likely to lose an additional limb.
When I DM/GM I always try to give my players options to get out of a bad situation. While it is fun to play a brainless bruiser from time to time, un-winable fights should remain that. You could always have an overpowered opponent due non-lethal or taunting moves to showcase there power and knowledge of their position in combat (After all what who doesn't love the bad guy that humiliates the hero? Doesn't it make the desire of growth and the subsequent later showdown and victory that much sweeter for the hero(ine)?)
Balance in all things.