NOOOOOOOOOOOOOOOOOOOOOOO

[Mervi]

So what happened in the episode? What I have seen and heard is Jack has been chosen as Jacob's successor, the characters in the alternate universe are up to... something, and Ben has apparently killed Charles Widmore. But where's desmond in the main universe if he's not in the well?

Yes, Jack volunteered to protect the island - it was up to him, Sawyer, Kate and Hurley to decide who it would be, but they didn't even get to discuss it before Jack volunteered. Hurley just said "glad it's not me".
Yes, Ben killed Charles because fLocke was promising him (Widmore) he wouldn't kill his daughter if he told how he was going to use Desmond. Ben said "he doesnt' get to save his daughter" and shot him.
Where Desmond is doesn't get answered until in the finale.

I hope that about covers it, but mind is still a bit bludgeoned after the finale so I can't really remember what else happened in "What They Died For".