I understand the basic principle that stirring batter (or kneading dough) develops the gluten in flour, and that how much you mix any given batter has to do with how much you want the gluten to develop. So, for quick breads like waffles or banana bread you want to stir as little as possible to ensure the most tender result.
I always assumed this rule was true for cakes and other baked goods as well, but then I made a recipe that called for the cake batter to be beaten on high speed for 2 full minutes. It came out moist and tender even with all the beating.
So what's the deal? How long should you beat different kinds of batter? When must you be really careful not to overmix, and when can you mix away? Why would you beat one cake batter on high for 2 minutes, when other cake batters should only be minimally mixed? What are the factors you should take into account if a recipe isn't specific about mixing technique or time?