I am new to this site, but love reviewing code and having my code reviewed.
One thing that is always difficult for me when I'm reviewing code is to determine whether some of the points I'm making are just my opinion or are objectively better.
I would prefer to make those points anyway, while noting that they might just be my opinion. Especially because not everything is equally subjective/objective and there wouldn't be much left if I would ban out all traces of opinion.
I can imagine though that those kind of comments can spark long discussions if there are no guidelines about how to handle them (e.g. the extreme case of tabs vs spaces).
I can't imagine this issue not being handled somewhere in another question on Meta or in the FAQ, but I couldn't find it, so I've taken the risk of posting a duplicate.