KVetch
Key Player
That's a better explanation than I could give. Another interesting feature is the formula does not take in to account who is taking the shot or who the keeper is. So Zlatan and Messi's expected goal would be the same as Sam Clucas for each individual kick.Exactly, but expected goals for and against give you more information.
Examples:
A team creates a lot of high quality chances, but fails to score. = Buy some better attackers.
A team allows only few high quality chances against, but concedes a lot of goals = Buy some better defenders.
A team doesn't create a lot of chances and at the same time allows many high quality chances against. = Sack your manager.
And I think different analysts use slightly different formulas. It's always being fine tuned.