I see so many people chewing with their mouth open, smacking their lips, talking with food in their mouth. As we become less formal about where we eat (at our desks, on the subway), manners become even more important. More people are able to watch us eat, and they definitely notice our bad habits.
A guy in my office sounds like a dog as he smacks his lips, and he talks with his mouth full, even on the phone. I am sure he is unaware of it, but everyone else can't help but notice. It has made me strive to have impeccable manners, so no one can have a similar complaint about me.