If you think stupid people are stupid, you are judgmental. If you think ugly people are ugly, you are judgmental.
It has adopted a negative connotation as the practice of paying any unbiased attention to behavior pattern of others becomes taboo. It's only socially acceptable to acknowledge all successes and instantly forget all failures, that way no true judgement of a person's behavior pattern can be formed. If people weren't afraid to actually tell when others were incompetent, much of modern society would unfold.
A: <says something really really stupid>
B: <makes WTF-face>
A: "OMG I hate you- you so judgmental."
C: "Yeah wtf, A, learn some manners."
A judgmental person is somebody who thinks they know everything about you when they really don't know shit. Judgmental people basically just live in their own little bubble of delusion and have no patience for the superstitious nonsense formally know has "open-mindedness".
Being judged pisses me off way more than anything else. Oh yeah, judgmental...