Actually there is such a thing as a bullet proof vest. Medieval armorers would fire a round at their armor, in front of the customers, and the bullet mark would serve as proof that the armor was effective, hence the term 'bullet proof'. It did not originally mean that no bullet could penetrate the armor, which is of course impossible.

Of course, since modern armor degrades significantly after being hit, a bullet mark only proves that you should use a different vest, so 'bullet proof' doesn't exist anymore.

Not really true about joints.
Curtain isn't only solution.
Dragonskin body armor is designed in scalemail pattern that allows better flexibility
(and before people start booing me, I am not a fanboy, I know that there were issues- glue was unreliable, total weight was higher and repair is pain in the ass)
But a concept of scalemail pattern for joints is something worth looking into.

They seem to be overlooking the obvious answer for the melee question: Spartans are obscenely strong, and once the shields are down it's not terribly difficult to do a very powerful melee attack targeted at the weak points in the armor. The assassination animations in Reach and beyond demonstrate this pretty clearly, and an elbow strike to the face (especially an armored elbow) is pretty frikkin dangerous.