A lot of this what I like/ what I don't like discussion can be seen as about objective vs. subjective.

"I love Avatar because the Navi people's way of life is so wonderful I wish I could just go there and be a blue person!"

-Totally subjective. While a bit odd, there's not clear way to argue with this, nor does it need to be defended. It's an opinion.

"I didn't like Avatar because the action was just too over the top, and the pacing was slow."

-These are mostly subjective but perhaps debatable.

"I hated Avatar because the plot was so plainly deriviative"

-While one could counter with "Well, I didn't mind the deriviative plot" this is a fairly objective statement.