Pay attention to Vader after he dispatches Palpatine; he is barely able to move, and needs Luke's help to get to the hanger bay. Easy to write off as injuries due to taking the brunt of Palpatine's lightning attack. Episode 3, however, reveals that all of Vader's limbs areare, at this point, artificial, and electrical...easy to short out due to an electrical overload by, say, sith lightning? Vader needs help to move because he literally can't at that point. He also knows that he'll die even if his mask stays on because, again, the life support system has been blown to hell.
In addition to the above, Vader's helmet has symbolic significance in that scene. Every other time we have seen Vader, his helmet is shiny and impeccably polished (for a prime example, see the scene near the beginning of the film where Palpatine arrives at the Death Star). Soon as Vader loses his hand and begins to see Palpatine torturing his son, the veneer is lost. Probably, the impeccable polish represents Vader's loyalty to the Empire, which prides itself on trim neatness and order, while the Rebels are the ones who are dirty and smudgy (their ships, their uniforms, their way of life in the original trilogy). Dirtiness represents good in the OT, so Vader's helmet gives away his intentions as he sees his son brutalized by his master — he is about to abandon the Dark Side.
Also, in light of Revenge of the Sith, the line "Let me look on you with my own eyes." is more meaningful since the LCD of Vader's lenses were red and black, the two dominant colors of Mustafar, where his life changed forever.
A frequent point of ire among fans is that all the Imperial officers, including Admiral Piett, have the exact same rank insignia (that of commander). Interestingly, this has not been addressed by Lucasfilms as a goof, meaning there would be a reason for this. Now, the Death Star II is being constructed in total secrecy, so the chance of leaks must be minimal. In real-life militaries, officers often remove their insignia when going into dangerous areas to not identify themselves as high-profile targets. By giving all high-ranking officers the same insignia, any spies or snipers would have to memorize what Jerjerrod, Piett, or any other significant personal look like and thus not just go by their rank insignia.
The Rebels recruiting the Ewoks as allies and the Rock Beats Laser is a bone of contention. Once you see the Star Wars Expanded Universe, however, it makes a lot more sense; the Empire has always run on Fantastic Racism, dismissing anything that isn't human (and/or Sith species, depending on the era) as unfit for anything other than slave labor...in a galaxy with over twenty millionknown sentient species. Meanwhile, the Republic (and the Rebellion, which is a de facto Republic remnant) always prided itself on being inclusive to any sentient species willing to abide by its laws. The Republic once again exploited one of the Imperials' glaring blind spots by actually dealing with the locals instead of ignoring them and bulldozing their homes. And as any Old Republic player who's seen Treek in action can tell you, those Ewoks are vicious and creative when they want to destroy something.
For much of the first half of the Battle of Endor, the Rebellion and their Ewok allies are actually being curb-stomped severely, both on the ground and in space. The entire fate of the Alliance is at stake, and things are not going well. And then on the Death Star, Luke finally begins to fight back, at which point the tide of the battle turns. This isn't coincidence: Luke's part of the Battle of Endor isn't just one of life-or-death, or of trying to redeem his father. It's the battle of Light and Dark itself. The moment he fights back against the Emperor, the ultimate representative of the Dark Side, is the moment that the Force is finally and fully with the Rebellion, helping to turn the tide of battle. The fall of the Empire was ultimately the will of th eForce.