I think this issue should be approached on case-by-case basis and use common sense. I try to avoid spoilers in my reviews, but in some instances they are unavoidable. For example, when reviewing the episode of the series which begins with resolution of a cliffhanger, you automatically spoil the plot for anyone who haven't watched the previous episode.

And there are also issues of whether thinking about spoilers make sense when referring the films or TV shows that have been there for decades and are part of general culture. Should referring "This is the beginning of beautiful friendship" in Casablanca count as spoiler? Or "I am your father" from certain film I won't name?