The combat system I'm working on (think mostly D&D-esque) has a concept of "Out of Guard" - essentially, situations where you're denied your defensive bonuses from things like Dexterity or a Shield. Its somewhat analogous to Flat-Footed from D&D 3.5.
But thinking about it a bit more... there's a lot of situations where you're Out of Guard. Running, casting spells, using a ranged weapon, being surprised... really the only things you can do and not be Out of Guard are make a melee or thrown weapon attack or walk.
So.... maybe Out of Guard should be the standard and having your Guard Up should be the exception tied to certain actions? There's also something to be said for getting to add a bonus instead of taking a penalty.
From this description, which way sounds more natural? Easier to keep track of? More appealing?