Sure, when the original three Star Wars movies came out geeks thought Vader was cool and kids were afraid of him, but nobody was really invested in his character arc until RotJ and even then people only cared because the hero's success depended on Vader turning away from the Dark Side (spoiler alert).
Hell even when they showed us Vader without his mask on (in the prequels) it was a total clusterfuck because the audience had a laundry list of preconceived notions of who Vader should be/what he should look like and that list was different for each audience member. Adapting a non-film/TV franchise to the big screen is hard enough, doing it with a main character who says about six different things every three years and never emotes is practically impossible.
I'm not trying to say that you can't make interesting characters without showing their faces, I'm saying it's very hard to get an audience to sympathize/identify with a faceless character. In 2012, if you want to make a blockbuster sci-fi action extravaganza you either need to be named James Cameron or Ridley Scott OR you need a recognizable and marketable ID figure. If Halo 4 smashes sales records that's a guarantee that there will be a Halo film at some point, it's just going to be hard as hell for 343 and their studio partner to make it both financially and creatively successful.
Not saying I'm the greatest writer here, but I think it wouldn't be too hard to make a Halo film trilogy and make it interesting and get people invested in the Chief. The issue with translating the games would be that you can't focus on the Chief's external action--you need people for him to interact with.
I think the guys at Ascendant Justice did an admirable first draft of a potential movie, but disregarding that they were absolutely right that it makes no sense to make the movie The Fall of Reach. It's Combat Evolved first all the way (although I'm not sure how you would handle the 2-3 transition. Would people be more accepting of a cliffhanger in the movies? I really don't know why it gets so much flak when people swoon over the Empire Strikes Back.)
On your point, I think the failure of Anakin and Vader as a character is there was a massive disconnect between them. The prequels were just horribly written and to a less degree poorly acted. Instead of being a consistent and subtle fall to the dark side Anakin just did a massive belly flop in the third movie.