Why isn't Knight In Shining Armor played straight anymore?

Well, I was thinking more along the lines of movies and television than literature, yeah.

The Iron Man movie didn't have any problems with it...

I only saw the sequel, but there they got around it by just having him open up his helmet (exposing his face) for no good reason pretty much all the time.

I was referring to the scenes where the camera went "inside" the helmet and gave us shots of Mr. Stark and his face while they did stuff. I wasn't particularly fond of the trend towards just opening the helmet, personally, but at least they never did it (without a good explanation) within combat. Still, I mean, the camera going inside the helmet (they do it prominently during the suitcase suit fight) worked fine, and he got to keep the helmet on...

There's that scene towards the end of Return of the Jedi where Darth Vader is deliberating over the potential death of his son. No facial expression; you can feel the emotion through the mask. Of course, in text-based media, this isn't a concern.