One of the things I've been mulling over about effects like "tired" or "injured" is whether they should have a binary impact, rather than a numeric penalty.
I've always disliked the strictly-defined effects that crept into RPGs (including late 1e AD&D and subsequent editions. Stuff like "-2 to this" or "-3 to that" or even worse, "-3 to this, -1 to that". (or the beneficial equivalent, such as feats.) Because of course you have to look up all that crap. To my way of thinking, it's not worth the effort in a game. What I want are rules that make stuff happen, but I don't care about rules that precisely distinguish between different degrees of penalty. As a result, the three-stage effect system I've been talking about in this blog for a couple years went from "all penalties are -1 at the first stage and -2 at the second stage" to "all penalties are -1, but the first stage is brief while the second stage is extended."
But what I'm thinking is doing away even with this explicit rule. Like any situation, if being injured, tired, confused, frightened, or suffering some other effect seems like it would be worth a penalty (or a bonus,) it is. GM's decision. But the only guaranteed effect would be to prevent or allow certain actions, or to trigger various events. Thus, an injured hand can't be used until the injury is seen to, while a very injured hand can't be used until healed; an injured leg forces the victim to hop (treat as max encumbrance;) an eye injury blinds, a stunning blow prevents actions and movement.