What should I comment on when contacting a potential advisor?
It's good to look into a potential advisor's research. If there is a specific reason you are interested in working with them, that is also good to mention that (briefly) in an initial email. 

However, I think making detailed, specific comments about the professor's papers would usually be a bad idea in this context. It is far too much detail for an introduction, and if you are doing it just to demonstrate your knowledge and/or interest, it will come across as forced and artificial. If someone was advising you to do this, I don't think it was good advice.

Keep the initial contact brief: introduce yourself and say something about why you want to work with them.