Are all adjectives opinionated?

A second standard English teacher must right now be telling her class, "An adjective is a word that describes a noun. There are four types of..." and she goes on.

'Sentiment Analysis' (as an earlier post of mine says) is a computer technique (a very layman way of calling it, actually) which tries to predict whether a sentence has a good opinion of something or bad. 

So, my question is, do adjectives describe, just as the English teachers of our country say? Or are adjectives opinionated?

When I say the film is good or bad, the adjectives 'good' or 'bad' are more opinionated than descriptive. What about 'the film is long'? Is 'long' an opinion or a description?

Intriguing. A description always tries hard not to be an opinion. When I describe something I don't like, more often than not, I will be projecting the negative side of that thing. So, a description at  the end becomes an opinion.

So, on the word level, it is very difficult to distinguish whether an adjective is opinionated or merely descriptive.