Spoiler free (essentially) for TV-only fans.
Joffrey is dead. But does it really matter who killed him? Of course we book fans know who killed him, right? The books eventually did reveal the culprit, but is there any reason that Martin and directors Benioff and Weiss are bound to the having the same murderer? A whole host of people had reason and opportunity to kill Joffrey. TV-fans' leading candidates are (in order of appearance)
Peter Baelish - in order to foster chaos from within he can seize more power
Tywin Lannister - in order to install the more pliable Tommen as king
Olenna Tyrell - in order to protect her grand-daughter from the monster Joffrey
Oberyn Martell - because he hates Lannisters and he ain't called the "Red Viper" for nothing
with other people speculating Sansa, Margaery, Tyrion (yeah, him!), and even Cersei. Among them, would it actually matter who did it? The future plot of the books is not driven by who did it, but who believes what about who did what. As far as I can tell, Martin, Benioff and Weiss are completely free to play a huge switcheroo on book fans and make us eat some crow when our book-smugness backfires on us by having a different killer on TV vs the books.
So here is to hoping that Martin plays us book fans like this. It would be a great way of emphasizing the Littlefinger/Varys debates about the nature of power and truth - that they lie where people think they lie - and would just be a plain old fun way for Martin to surprise us yet again.