Just as a person in a bulletproof vest who gets shot will get a few broken ribs, stormtrooper armor does prevent the trooper from actually getting killed, though a blaster shot will take them out of the fight. Read more about the armor and you’ll see that it doesn’t keep them in the fight, it merely keeps them from being killed. Or, since you probably won’t follow that link,
“Although standard armor lacked the durability to make the wearer immune to cannon fire, it could partially deflect or disperse energy from low, medium, and high-energy blaster bolts; though the wearer may be incapacitated, survival would allow the trooper to receive advanced medical treatment that may return him to service. It also deflected stun beams, and served as excellent protection against explosions and shrapnel, thereby drastically reducing the effective casualty radius of fragmentation weapons used against troops wearing such armor. It was almost impossible to kill a stormtrooper with a slugthrower unless the bullet was abnormally large, specifically armor piercing, or if it hit the body glove or visor lenses.”
or
“Stormtrooper armor as portrayed in the films is highly ineffective as combat protection and is generally hindering to the wearer. It largely contributes to the now infamous stormtrooper effect and has given rise to much fan speculation and outright mockery. However, this is countered by numerous role-playing, and other such sources, wherein stormtrooper armor is, on average, the best available armor, for protection versus weight/mobility restriction, and advantages, only being limited by its illegality to purchase, thus commanding exorbitant prices on the black market. ”
As for physical protection, knocking a person out is a matter of spinning someone’s head fast enough for their brain to hit the inside of their skull. Short of a neck brace that would severely limit the trooper’s field of vision, there is no protecting against that sort of thing. The helmet may keep your skull from breaking when your speeder bike hits a tree at high speed, but no physical armor protects against G-forces.
Also bear in mind that most conflicts a stormtrooper sees are against normal citizens who are less heavily armed and far less trained than the elite people of The Rebel Alliance. Riot police have a better chance against civilians throwing rocks than they would against SEAL Team Six.
Oh, let us not forget that Han Solo used to be a stormtrooper and thus would know the weaknesses of both the armor and the hand-to-hand training of Imperial troops.