Even quasi-ambiguous statements like "Wow, plot twist!" or "Hmm, the acting was sub-par," are spoilers after a fashion. Just wrap your sentiments in spoiler tags to be safe, even if you think they're harmless.
It sounds like common sense, but some people will get excited and forget; others might just think it's funny to trick people into reading spoilers. It's really not. Some of us have work/exams/etc. and won't get to see it for a couple days at least. Please be respectful and help us avoid spoilers! We can all discuss it together in a couple weeks, but until then let's help each other out.