The change that caused the controversy was a bug fix for Pantheon's ultimate move Grand Skyfall. Grand Skyfall lets him leap into the air and then slam into a target location, damaging and slowing any enemies near the point of impact. Pantheon used to be able to cast spells while landing. This proved to be a very powerful combo move.
However, Patch 4.7 prevents Pantheon from casting until he's back on the ground. On a related note, they changed the animation for Grand Skyfall so the animation and splash damage line up better.
"In short, we were looking at Pantheon's Grand Skyfall animation and realized it was inconsistent from a visual standpoint (ie: almost getting out of the circle as Pantheon lands, only to have him land while also stunning you in the same frame). We'll be keeping an eye out if this makes Pantheon feel less smooth to play than on live (not from a power perspective but from a feel perspective!)."
While Riot considers this a bug fix, some players deemed it a nerf to Pantheon. The fact that the change wasn't documented in Patch 4.7 at first probably didn't help their temper. The timing of the change was also deemed suspicious by a few players; Riot began selling a Dragonslayer skin for Pantheon right before this bug fix was announced, leading players to spend money on a champion now weaker than before.
Players' rage over the change led some to make death threats, according to Riot player relations specialist Chris Tom.
Death threats for Pantheon changes. #seemslegit. I understand the passion, it's what I signed up for. That said, I do get concerned for...— Chris Tom (@RiotPwyff) May 17, 2014
... the designers who just wanted to make a deep and engaging game. They didn't sign up for that abuse so it's a little hard to watch :(— Chris Tom (@RiotPwyff) May 17, 2014
I'm not saying "GO YELL AT THE HATERS," but designers just want to make games. They communicate because they like talking to players.— Chris Tom (@RiotPwyff) May 17, 2014
Gamers are allowed to complain as much as they want. If I played Pantheon regularly, I'd be upset too. If we could keep the discussion at a notch above "I'm going to murder you" on the Civility Scale, though, that'd be great. I'm pretty tired of reading about this crap.