The latest update to Diablo III appears to be something of a downgrade for many players of the game. It's being widely reported by users on the game's official forums that a bug in the latest update blocks players from getting any experience points and level-caps their followers at level 13.
It sounds as though the bug affects only those with digital copies of the game—I installed from a disc, and haven't been able to replicate the problem with either my PC or Mac versions of the game.
Fortunately, it looks like there's an easy fix. Based on the chatter in general chat in the game, and backed up by the many posters in this thread, it would appear that you have only to change your password, and you'll be able to gain XP once again.
As I said, I can't confirm this with my own copy of the game, but based on the number of responses in the original bug thread and the subsequent fix thread, it seems that this is affecting enough people that it's a legitimate concern.
So, an odd and potentially crippling bug with a simple solution. Still… it would be nice if Blizzard's constant updates to Diablo III made it work better, you know?
Kotaku has contacted Blizzard and will update once we hear back.
Update: A Blizzard representative has gotten back to us, explaining that they fixed the bug soon after it turned up:
After downloading patch 1.0.3, some players ran into a bug that temporarily caused them to stop gaining experience points and capped their followers at level 13. We quickly fixed this after discovering the bug.
EXP Bar [Bug Thread, Battle.net]
Quick Fix for .3 Update [Fix thread, Battle.net]