EzekielRaiden
Follower of the Way
4e specifically called it "bloodied," and explained it more or less like this. Before "bloodied," you might get scuffed up, but you hadn't really taken much outright injury yet. It was actually extremely useful to have this as a status, because you could tie all sorts of useful things to it. For example, I remember vividly a fun boss fight I went through in a 4e adventure where GREGOR the strange, strange man attacked us, and he went through four different phases--initial, post-bloodied, transformed after being "killed" and revived as a revenant, and bloodied as a revenant. It was a delightful fight, both mechanically and narratively (the DM gave GREGOR a delightfully scenery-chewing personality.)i personally interpreted it to mean, or at least thought it made more sense to mean that half health is the point where it starts becoming blatantly obvious that you're battle-worn to some degree, like here's a poison/sickness example:
some days you're ill, your body is just off kilter and not feeling quite right, fundamentally you're at less than full HP but you still look to the casual observer pretty much all okay, but someone who takes a closer look and you're more pale than you usually are and your hands are oddly cold, then half health is the point where you're visibly clammy and your eyes are all red.
above half health you might have a nick or bruise or two, a little out of breath and cheeks are kinda pink, past half there's a slice on your side that's bleeding, you're holding your arm at a conspicuously not-relaxed angle and not moving it in a certain way, you're breathing heavy and your face is red.
Edit: And this, incidentally, was the meaning behind Healing Surges and the like. There's only so far you can push yourself, so much you can draw from the well of resilience and determination etc. Once that well is dry, combat is incredibly dangerous and you should really stop. Combat becomes VERY lethal in 4e if you have to fight while out of surges.