Design students/professors: How do you critique?

"Question for everyone: How extensive/thought filled are your critiques that are conducted in class? Two parts to this question: ...The reason why i ask this: In my classes, i've heard a lot of students unable to speak their mind. I hear a lot of 'cool' 'i like it' 'i don't like it' all mixed in with the word 'like' about 20 times in a sentence. The critiques are often lacking substance, and teh students don't back up their opinion."