My answer is very similar to Tree's, especially this quote: "I also have this thing where I don't recognize my hands...they look like they belong to someone else. So when I am under stress I try to not look at my hands very much. Eeek. "

That said, however I actually stare into mirrors for long periods of time. (which is TOTALLY different for me than those dratted "mirror excercises" that groups use grrrr). but my face is different so often (and has been so all my life) that I am always deeply fascinated.
My face never seems the same twice in a row. Often i do recognize different ages like 16 ... but there is just as often that i don't know the face - so i will stare into the eyes to see if i know me. Or lay on my bed and stare at the closet mirror to see who i am today.
Taller, shorter, hair lenght change? Eye color change, facial features change, eyes wide open or slitted or something i've never seen before? Sometimes it is my dad's (abuser) face and i do look away quickly. i hate that face.