In my opinion, some of the most memorable and defining moments in D&D come as a result of those split-second decisions that players must make about how far to risk their characters' life and limbs. You know what I'm talking about. Your party is being decimated by a pack of ghouls. Grumple the dwarf has been cut off from the rest of the party, trapped in a room with four ghouls closing in for the kill. He's wounded and will surely die if left to his own devices. Meanwhile, the other three grieviously wounded party members have an escape route - 50' of open hall in front of them - beckoning...
What sort of thought cross your mind in a situation like this? One factor in this split-second decision is "how much do I value that trapped PC?" I don't think heroism is applied on an equal-opportunity basis. This is where a mercenary mentality comes into play. D&D players are generally smart if nothing else, and I assure you it makes a difference whether it's the new 1st level recruit trapped in the corner or the proven and battle-worthy 4th level dwarf! I'm sure many of us would quickly assess the value of the trapped individual - is he worth saving?
Alignment may be another variable. A player with a lawful paladin may rightfully say "I will not leave my friend at the hands of evil underworld denizens! With God at my side - I will fight!" A player with a chaotic thief may say "Hey man - I love you dude - but I gotta go!" And the entire time, that thief may be thinking "I'll come back later and rob the corpse of his gems and magic sword." Alignment can be a very appropriate input into the decision.
Perhaps most importantly, some players simply have a "Heroism Value" which dictates their character choices. They have the attitude that they're going to play their PCs in a heroic way, like Marvel superheroes or Conan the Barbarian! After all, would Captain American flee a losing battle and leave his peers to die? No way! Would Conan run away and leave a maiden at the hands of evil cultists? No way! (Well, he might consider it, but he wouldn't actually do it). By the way, I'm not implying that this heroism value equates to sure and certain suicide. But it certainly means that life and limb will be risked in fairly dire situations - because that's what brave fantasy adventurers do!
Opposed to the "heroism value" is the "Risk-Reward Value." Many players quickly do a risk-reward calculation, assessing the entire situation and the likely outcomes. They then decide what decision to make. They consider how much time is invested into their PCs, how valuable the cut-off party member is, how much gold and bling they have earned and stand to lose if slain. Hell, they may even quickly assess whether they have access to a Raise Dead spell back in town. And most important - what % chance does their PC have of dying if he races back to help his comrade? Based on all known variables, they act accordingly.
Of course, heroism value vs. risk-reward value also manifests itself in the little decisions that come about during dungeon delving. Is it always the same player who says "I'll open the chest"? Is there always a certain player who is never willing to explore that little 2' wide wormhole twisting off into the ground? Are there certain players who let their PCs hang back from combat if they're wounded and let other characters take the brunt of the fight?
I'm not implying that either style of play is preferable. In all honesty, everyone should have a little bit of both styles of play. If I have a player who is seriously wounded, I should hang back - as long as everyone else is fully healthy. But I shouldn't hang back if we're ALL badly wounded, just to let others take the blows.
As a player, I tend toward trying to play my character as heroic, willing to risk life and limb. To me, it's simply more fun that way.
What about you?