Colour_ This is why wearing clothes that are faded isn't a good idea, it may not seem like a big deal to you,but people actually notice.
Fit_ they notice if the outfit is too tight or too loose. You don't want to be addressed as "that guy that wears oversize"
Pattern and prints_ for some people, its easy to recall a popular pattern/print of any outfit anywhere they see it. So patterns and prints are very noticeable.
The state of the outfit_ if its clean or dirty, wrinkled or ironed,people notice that too. Its advised to always look presentable and smart even on days you just Want to be sloppy
How you pulled off your entire look_ this may sound unbelievable, but people do notice how you pulled off your entire look. How else do they come up with stuffs like "that lady can do colour riot".
What's the first thing you notice about someone's outfit when you walk up to them?