"Ian McKellen was really awesome in that movie, but he's never gonna win an Oscar for it because it's rubber mask theater."
"Rubber mask theater" is slang for any movie or television show in which some or all of the actors must emote through layers of latex or silicone prosthetics and heavy makeup. These productions are typically genre-based and deal with science fiction, fantasy, or horror topics. As such, the actors also often have to interact convincingly with puppets and react to CGI and special effects that they can't properly perceive as they work. Recent, classic examples of rubber mask theater include Farscape and the Lord of the Rings movies.
No matter how well written, acted, and directed such works are, their genre nature guarantees that some critics will never consider them to be "serious" works worthy of true recognition. (On the plus side, such works often do very, very well at the box office.)
Thus, actors who work in such films not only have to contend with the physical challenges and discomforts of working in makeup and appliances, but they have to deal with prejudice against them and their work. An actor has to turn in a really astounding performance in such movies and shows before he or she will be critically recognized with award nominations. Some performers, such as Andy Serkis as Gollum in the aforementioned The Two Towers, are overlooked despite the extraordinary nature of their work. Contrast this with movies such as The Green Mile which, while strongly (but subtly) genre-based, lack overt "rubber mask" trappings and garner plenty of critical respect for their actors.
It's all a little like the distinction between fantasy and magic realism in books. Straightforward modern fantasy is inevitably considered to be juvenile, commerical trash worth no serious notice by the Great American English Departments, whereas magic realism is often hailed as important literature. | 
