VAGUE SPOILERS FOR THE ANGEL FINALE!
The end of Angel was very deliberate. It wasn't a case of "we'll resolve this next season...oh, I guess we won't." They were trying to say that the fight continued and always would, that there really was no end to the struggle.
Here, I'll let Joss say it (from Wikipedia):
"But the point of the show is that you're never done; no matter who goes down, the fight goes on." Whedon says that Angel is about redemption, "something you fight for every day, so I wanted him to go out fighting. People kept calling it a cliffhanger. I was like, 'Are you mad, sir? Don't you see that that is the final statement?'