Fascinating to read the replies.
I guess colour would be a big one, followed by shape/proportion/fit.
I do notice shoes and statement jewelry. I also notice hair and nails.
I don't tend to notice regular jewelry, handbags or brand names (ie types of jeans, designer shoes etc)
If people watching it is often the ease of the look, and the appropriateness, or if it is something I think I can poach or something I would love to wear and could not. I remember a gorgeous melon playsuit last summer at the races, it was so fun on a young 20 ish year old (but would not have been right for me at all). I remember seeing a woman with a stunning red blazer and straight jeans and converse several years ago, I tried to copy that one as soon as I could.