In my high school English class, which was the honours class, we learned that a clause starting with which always takes commas. Clauses starting with which contained non-essential information and were set off by commas, and clauses starting with that contained essential information and were not set off by commas.
We even got a hand-out about this:
In my current grammar class, three years later, the textbook is saying that a restrictive adjective clause starting with which is OK. That is, an adjective clause starting with which does not need commas. This I disagree with.
So on my assignment, where I had to write a "restrictive adjective clause containing which functioning as object of verb" I wrote:
The assignment required me to use a concept which I dislike.
Yep, I'm passively-aggressively arguing against a grammatical concept by means of my example sentence.
The sentence I wrote looks wrong to me. "The assignment required me to use a concept that I dislike" looks and sounds much better.