Watching both Supergirl and The Flash this week, I got to thinking about the difference between feeding information on an ongoing plot (Supergirl) and the cliffhanger (The Flash). There will be spoilers ahead, so get out now if you care about that.
(Actually, it's interesting to see where the shows are similar and they differ, because both deal with similar themes and have an overlapping executive. Barry is pretty consistently dealing with father issues, while Kara is dealing with her mother and her aunt. Both of them have collected this surrogate and not so surrogate family. End digression.)
My first thought was to say that one resolves the main plot and one doesn't, and that's certainly a big part of it: Supergirl beat Reactron, but the Flash didn't beat Zoom. We could have easily made the Flash episode into not-a-cliffhanger: Barry beats Zoom but pays the price of his powers: we've resolved the main conflict but made the main character pay a price.
Now, not resolving the main story isn't the only way to go. You can resolve the story but the new information about any plot kicks it into high gear. The audience wants to know what happens next. A twist where they suddenly see things in a different way but where there's nowhere to go is a twist ending. The revelation at the end of The Sixth Sense closes things off, rather than opens them up.
So it's a cliffhanger if they want to know what happens next. When you dole out information about, say, a conspiracy that the Girl Scouts are involved in (thank you, JLU), it remains abstract.  But when it's information about a person that the players know, it is much more likely to become a cliffhanger.
Or have the information be a context shift for the character. Barry's words at the close of The Flash are a fine cliffhanger, but the same sentence spoken by, oh, Joe West wouldn't have the same impact.
1. Memo: Find out whose job it is to add fluoride to the water in areas with fluoridated water. That's the guy your supervillain wants to coerce or blackmail in order to poison the city's water supply. Even if the water flows over a bar of stannous fluoride, someone has to put that bar there.