UPDATE: Tweaked to reflect what's actually changing with Shadow Fiend. Thanks!
First and foremost, Shadow Fiend has been dealt some changes, mostly aesthetically. Here's a summary of where he's (it's?) at:
There's an entire page that demonstrates the new and improved Shadow Fiend. Coinciding with Shadow Fiend's update is the availability of an arcana item set, which grants a different skin, voice, attack effects, and other ways of making your character stand out 'n look pretty.
Other changes include courier morph (which means your courier will be temporarily visually displayed), the launch of fantasy (a la fantasy football) season two, fight recap (providing a detailed statistical breakdown of matches), a new location for Roshan on the map, tweaked versions of Bloodseeker and Phantom Lancer, and a bunch of other changes that are better seen by reading the bottom of this page.
Because I don't know anything about Dota 2, I asked our expert, Mr. Shoemaker, for quick thoughts:
"This is the most radical stuff they've done to the fundamentals of Dota 2 since it came out. They're the kind of changes normal people would look at and roll their eyes--stuff like a handful of trees being in a different place. But changes like that just keep raising the skill ceiling because they give really good players a ton more options for ways to maneuver and strategize. The nature of these changes is tantamount to changing the length of a football field or the duration of a shot clock in basketball, except there isn't just one change, there's a ton of them."