I have always loved to write dialogue and I really got to play with this in the Phantom. Painting a picture of a character through their words or actions is something I feel like I do well. I've done the 'looking-in-a-mirror-describing-myself thing and felt horrible awkward doing it. I'm like that in person, too. Like, I describe my friends and family by who they are, not what they look like, and it comes out this way in my writing as well. I have often found myself lost for words (however briefly) when asked, "yeah, but what does he(she) look like?" To me, I guess that part doesn't matter so much, unless it's an unusual feature that sticks out.
It's both a blessing and a curse, because I've had readers do both, on one hand thanking me for leaving room to imagine what somebody looks like and on the other complaining that they never know what anyone looks like.
I dunno, it just feels foreign to me to say, "he's 6 ft 2 in and 200 lbs with brown hair and eyes and a mustache." It's way more comfortable to say "he's about the most laid-back person I know, smiling and throwing jokes around, playing like someone half his age." So, that's the way I write.