Title says it all. I've heard scores of arguments FOR guns in D&D... now I'd like to hear from the other side of the fence. What is it about guns that just screams "NO!" in your campaign worlds?
1) Tone. Flavor. Lore of the setting. Fantasy Gun Control. The flavor of guns and other "high-tech" items (even though early guns were
not high tech) doesn't fit with most fantasy settings. Even fans of relatively high-tech settings like FR (where guns actually exist, although I believe they're from Starjammer) and Eberron resist having guns added to the setting.
Having magic available to only a small portion of the population, IMO, fits those settings better than having guns available to anyone who can afford or steal them. (Ironically, in FR, magic is all over the place, and in Eberron, anyone can buy a little magic from a store. Still, people can't afford Wands of Magic Missile unless they've got some kind of powerful sponsor.)
2) Rules.
Why does reloading a crossbow in DnD take such a short time, but reloading a gun takes so much longer? It's like, because guns are used in modern day combat, they have to take actual historical data and impose them on the game.
IIRC, you couldn't keep a bow strung all the time. If that rule were imposed in DnD, it would ... suck.
Warhammer Fantasy has more realistic rules, with long reload times for both crossbows and guns. No one uses a crossbow unless they have to, and I've seen players waste up to two full rounds trying to reload a pair of pistols. (Or is it four rounds? I don't recall.) And of course, there's the brace of
extremely expensive guns. Then again, Warhammer prevents you from using a longbow properly unless you have Rapid Reload.
On a similar note, one of my few beefs with d20 Modern are the myriad versions of guns. You don't have 2'-6" foot long swords doing more damage than 2'-9" swords in either d20 Modern or DnD, but for some reason people become consumed with muzzle velocity, caliber, etc when it comes to guns. (For my games, I ruled that all automatic rifles have the same stats. Not realistic, but I just don't give a damn.) I think this type of thinking carries over to DnD designers.