My newest car (which is still 5 years old) automatically adjusts the rear view mirror tint so that you don't get brights in your eyes.
Our car has that and I don't really like it much - sometimes it fails to dim when I want it to, and my wife said that it dims too easily for her liking. A mirror with a manual lever mirror worked better for us. (Our truck has it, too, but the cap over the bed with tinted windows means that headlights are never bright enough to trigger it.) It's not something I actively hate, but if our next car didn't have it, I wouldn't mind.
What I would like is a way for the headlights of oncoming cars to be blocked out. I sometimes do this with my hand and I can instantly see far better. Unfortunately a lot of people don't seem to understand our eyes and how light works - for example, a cop car with bright, flashing lights at night can actually blind your eyes to whatever they were trying to warn you about.