Your Players Aren't Heroes

This guy would probably rape your sister

A primary difference between new school and old school D&D, at least to me, is the concept of the player character as a hero.  In new school D&D (3e, 3.5, 4e, and even a lot of old 2e), characters are generally bound to some sort of storyline which will ultimately have them becoming heroic figures through their various adventures along the story path.  While there is certainly nothing wrong with this, it is quite a bit different than old school gaming.  In old school D&D, and games built around old school principles, the character is not a hero; he/she is a mercenary and a conqueror.  Sure, he might perform a heroic deed, and true enough, most adventures may revolve around resolving some quest which by nature seems "heroic," but ultimately the PC in the old school game has three main goals: survival, killing shit, and taking their stuff. 

Think about this for a moment before brushing it off or dismissing it entirely.  In old school formats the brunt of your characters' experience is gained from living, killing monsters, and accumulating wealth.  Sure you might save a princess here or there, or save a village from a mad wizard, but you're also a tomb raider, and a sword for hire.  Let's face it, a rag-tag group of adventurers not bound by any sort of overlying story, are mercenaries.  They're hired to complete a task, which usually includes killing monsters (and sometimes other humans/elves/dwarves etc), and stealing the shit they've managed to accumulate.  Ultimately the character is seeking to get enough power to overthrow that despotic ruler, build a keep, and establish an army, only to eventually become the very despot and tyrant they conquered.  Some up-and-coming adventuring party is out there, just as you once were, and they're seeking to overthrow you, conquer your kingdom, and take your shit from you.  Because that's what it's all about in the old school.

In old school D&D you are an adventurer, mercenary, grave robber, and a conqueror.  As you continue to survive and build a name for yourself, its usually accompanied by fear among the populace, because you accomplish your goals through the blade at your side, or the spells in your spellbook.  Your character may be "good" in terms of alignment, but a hero he is not.  Survive, kill shit, and steal shit; these are the principles the old school player lives upon.  These are the only forces that guide his fate.