How important are subject lines in deciding whether to open a newsletter?

They are super important. That's the one thing that can double your open rate. Think about it when you open your email the next time. What captures your attention?

Subject lines are like the trailer of a movie. If the trailer is boring, you won’t want to watch the movie, right?

Can't be ignored, but reflecting on my own behavior, i think the subject line is more important for free newsletters than paid. If I'm a paid subscriber to your newsletter, I'm already (literally!) bought in to what you have to say. I'm more likely to treat free newsletters as a pool of options on my attention -- if I get grabbed I'm in.

people often judge emails by their subject lines, so a great one can really boost your open rates

I would say very important as with tons of newsletters to choose to read from, one that catches my attention matters :)