We've all been there before...
We set up expectations for a campaign: rough-n-tumble explorers, daring swashbucklers, epic heroes out for Queen and country, grim anti-heroes out for vengeance, gritty mercenaries adventuring for gold and glory (and ale and whores). We're seeking a tone, a theme, a certain feel to the campaign...
...and it doesn't survive char-gen.
Sure, your PCs may be rolled-up according to a specific list of choices, with a books-worth of campaign background to go off of. But once the adventure is afoot, the tone changes. Those noble crusaders search every last corpse for gold fillings and prefer to shoot first (at ambush, if possible) and ask questions later. Or those gold-and-glory PCs who should be dirt poor by next session carousing and wasseling have instead opted to pool their resources into buying magical items, a keep, or some other long-term investment. Or those explorers who spend 20 minutes "taking 20" on every. single. door. Or those anti-heroes are WORSE than the villains they face!
In short, what happens when the PCs abandon the tropes of their world and play according to "smart" play; using tactics like overwhelming force and guerrilla strikes; budgeting every gold piece, killing prisoners, or triple searching every 10' square for traps. Such tactics are smart AND effective in normal play, they're just the anathema to the genre of fantasy your seeking!
(and that doesn't even touch the wrong-genre adventurer; IE d'Artagnan on Hyborea).
Interested in hearing some stories.