So I've been playing a lot more Diablo 3 in preparation for this. I hadn't touched D3 since about a month after it's release, when my barbarian was struggling to survive in Hell difficulty (which isn't even the hardest difficulty).
Blizzard has made a ton of improvements to the game and classes since then. They were serious when they said they would retool powers/talents and buff the crap out of the lesser used ones until they reach the point where they become equal to or more attractive than the cookie cutter powers/talents. I was very happy about this, because the only build that could survive inferno at the time was a build I was not interested in (It involved using a shield. And that's sacrilege to my barb).
They've added paragon levels, so you can continue to get more powerful once you hit 60. For every paragon level, my barbarian gets 3 strength, 1 dex, 1 int, 3 vitality, 3% magic find, 3% gold find. There are 100 paragon levels to be had (allowing you reach the 300% magic find cap, so you can ditch magic find gear for gear with better stats).
They've also added something called Nephalem Valor. Every time you kill a champion or elite pack (but not unique, purple-named bosses) or every time you open a resplendent chest, you get a stack of Nephalem Valor which lasts 30 minutes. Every stack adds 15% magic find and gold find, up to a maximum of 5 stacks. The timer gets reset every time you kill a pack or open a chest. This magic find is on top of the 300% cap. But, at 5 stacks, it also guarantees a bonus rare (yellow) drop whenever you kill another pack or open a chest. Which means even more chance for legendaries (uniques).
They've also included Monster Power. In Diablo 2, you could type in a command in the single player game that would simulate having more players in your party (so that monster's had their damage and health buffed, but also had an even higher chance of dropping better things and the experience gained from killing them also increased). This is kind of like that, but it works even when you're in a party. Essentially you can increase the difficulty of the game but also increase your chances of finding better stuff. There are 10 levels of power to choose from. (0 being disabled).
They are making even more refinements to the loot system (that is already live with the console version of the game) that will either come out soon, or with the expansion. Essentially they are making loot that drops have a higher chance that it will be something your character will find useful. As well as guaranteeing a Legendary after every first major boss kill. On top of that, they are retooling Legendaries in general. Before, every Legendary had a level associated with it, and only monsters around that level could drop that specific legendary. With Loot 2.0, every legendary has a chance to drop from any monster, and the Legendary items will adjust their item level based on the monster that dropped it.
I've definitely been having a blast with this game again in the past few weeks. Where I was frustrated with my barbarian and my chosen build, I am now soloing Inferno difficulty on Monster Power 6 (I could push to 8 I think, but I haven't been that daring), and I
definitely don't use a shield.
If you were like me and enjoyed it for a time but got frustrated, I'd recommend giving it another shot. They've made big strides in improving the gameplay, and are only continuing to take it even further.