The 2.0.1 patch brings with it a huge list of changes, including completely revamped loot, difficulty, and class abilities.
Those of you who have left Diablo III unclicked since its less-than-optimal launch may want to consider firing it up for the massive new Loot 2.0 patch that it has just received. The patch, as always, is completely free to players, and does not require the Reaper of Souls expansion set. The highlight of the patch is the long-promised "Loot 2.0" loot revamp, which basically ensures that you'll get less loot, but it will be better, more epic, and tailored to the class you are playing (no more wizard hats with +strength). In the three hours I played since the patch was released, I found three upgrades for my character, two of which were legendaries.
As well as Loot 2.0, the 2.0.1 patch brings with it an absolute wealth of class changes, much too numerous to list here. Just know that every class has had new runes and skills added, and various other skills tweaked, or balanced. The Wizard, for example, can now make entire builds specialized around a single form of elemental damage (fire, frost, lightning ect). Read the full patch notes here.
There's also a revamp of the game's difficulty, which was previously broken into four levels (normal, nightmare, hell, inferno), and then made increasingly difficult with the addition of monster power. There are now five regular difficulty settings (normal, hard, expert, master, and torment ), with torment having a "difficulty slider" that goes all the way from torment 1 to torment 6. Each level offers new bonuses and rewards, visible in the difficulty selection menu, and you can now lower the difficulty in-game.
Paragon levels are now also global for all of your characters, and infinite, meaning players can constantly make improvements to their character.
I don't really know how to describe this patch in any way other than "amazing". As soon as they got rid of Jay Wilson, Blizzard has made nothing but sound decisions for Diablo III, and the culmination of those decisions is this patch, which simply focuses on making the game fun, as fun as all those hours I sunk into Diablo II back in the day.
And because I'm sure someone will ask in the comments, no, it hasn't removed the always-online requirement, and yes, the Auction Houses will shut down this month.