The latest Diablo III update has broken a few things, but there's an easy fix.
According to the game's official forums, numerous players running the latest update have discovered that they can't gain any experience, their followers are capped at level 13 and health globes have been rendered invisible.
The bug apparently comes with the v.10.3.10057 update and only affects those with a digital copy of the game. Fortunately, there's an easy fix, albeit one that doesn't come from Blizzard itself. According to player chatter, changing your game password will fix the issue, allowing you to start leveling up once again.
I fired up my digital copy of the game, and I didn't run into any of the aforementioned problems.
There's no word from Blizzard yet as to whether this a legitimate issue, but the numerous posts and confirmations on the game's forum seems to indicate this isn't a prank or some kind of super-elaborate phishing attempt.
Source: Diablo III Forums