SWSE also has a mechanic which kicks in at half HP, called 'second wind', which allows you to spend a swift action to regain some HPs after you reach that point.
It should also be pointed out that "half total hit points" is used as a game mechanic at least twice in the PHBII - the Dragon Shaman can only heal allies up to half hit points with his healing aura, and the Barbarian berserker rage feature kicks in once you drop below half hitpoints. Clearly this mechanic has been on the minds of the designers for awhile.
Given the way D&D abstracts hit points as "adrenaline damage" rather than, necessarily, actual injury, perhaps they're quantifying this in 4e, so that the 'top' half of your hit points are effectively just morale, and you only start taking physical damage (become 'bloodied') when below half. That could have interesting implications for injury-based poisons and secondary effects.
From the one game I've played (so far) of SWSE, I like the condition track, and see it far more as a substitute for ability damage or level drain than as a direct tally to hit points. In that context, I think it fits the D&D game model perfectly.
