I don't remember when all the changes took place, but the game is substantially different versus its incarnation at release.
1. Difficulty levels have been signficantly adjusted. Damage is far less spiky than it used to be. "Monster Power" levels have been added. So, within the Inferno difficuly setting, you can choose any Monster Power level beteween zero and 10. The higher the MP level, the more powerful the monsters, which correlates with more experience points and higher chances of better item drops. It is hard to say whether the game is now "easier" or "harder" than before, as so many things have changed. However, it is certainly more playable. Infinitely so.
2. Legendary items have been entirely revamped. The original Legendaries (now labeled "Legacy" items) were awful and never nearly as good as well-rolled Rares. Now, Legendaries are usually the best of any particular item you can get, by a wide margin. This is due to higher rolling numbers and more useful bonus abilities that synergize.
3. Classes have been significantly modified--vastly upgraded, for the most part. Abilities have been buffed to make them more usable. Certain exploits have been nerfed, though. The Witch Doctor has managed to become the most over-powered class in the game.
4. New account-bound super items have been added to the game. The first was the Hellfire ring. You have to collect keys by killing Key Wardens (new, super Elites). There is one Key Warden in approximately the same place in each Act. You need to collect one key from each (except from Act 4, which you only need to do once), to make one Diabolical Device. Each Diabolical Device opens one portal to fight an Uber Boss. There are three different Uber Bosses (well, actually six, as you fight two at a time in each battle). Winning an Uber Boss battle yields the chance of one component of the Hellfire ring dropping. You need three different components to craft one ring, and each Uber drops a specific component. The higher the MP level, the greater the chance of a component dropping (same goes for the keys).
5. In addition to the Hellfire ring, you can now pick-up plans to craft certain Level 63 Rares (gloves, shoulder plates, rings, amulets, etc.). These special, craftable Rares can roll higher than ones you find in the game. They are also account-bound (cannot be traded or sold). To craft one, you need to collect the plan for it and a certain number of Demonic Essences. Both only drop from killing Elites.
6. They added a half-baked PvP function, called "Brawling." You go to a single level, where any other player can join you. There, you just bash each other's brains out, to no consequence whatsoever.
7. A new top-level gem has been added, the Marquise. It's expensive to craft and cannot be traded or sold.
8. I'm not sure if this was the case when you were playing, but it has been for some time: Life Steal is now absolutely essential to play into the higher MP levels. At least one of your weapons has to have it--otherwise it is often impossible to survive an Elite battle. Life-on-Hit is now meaningless for the most part.