The KISS Rule
I think shotguns embody the KISS Rule better than any other weapon: Keep It Simple, Stupid! Attempts to make the shotgun a more battle-worthy weapon take away from its simplicity and speed.
If you want to knock a guy down quickly at 25 yards, there's nothing better. If you want to do more, you should probably have a rifle.
For nearly any civilian defensive scenario, you can't go far wrong with a plain-jane 12 gauge pump loaded with 00 Buck. The autos are fine, perhaps even superior technically, but I don't think the expense and complication translate into any realistic tactical advantage. They probably require more maintenance and training as well. The double barrels are OK if that's what you've got for legal or other reasons, but I think it's a good idea to have more than two shots before a reload.
The tacticool accessories should be left in the Cheaper-Than-Dirt catalog (although a weapon light is a good idea, actually). I don't even like the extended mags (source of stoppages).
Keep it locked up but loaded (full mag, empty chamber, safety on). As soon as you deploy it, rack one into the chamber and take the safety off. Abide by the four fundamental rules of firearm safety.