Essentially, Diablo 3 is a full-on action role playing game. It takes the tried-and-proven idea of the dungeon crawling concept and simplifies it with the excessive use of mouse clicks. Any veteran Diablo player will tell you how much abuse their mouse takes when battling the minions of the Underworld. (And the mouse replacement they made after many hours of game play).
The bread-and-butter of Diablo games lie in the unique classes that are available to choose from. In this third iteration you have the Barbarian and Wizard which have appeared consistently since the beginning, and then there's the new Monk, Demon Hunter and Witch Doctor classes.
If you love diving into the fray as a hand-to-hand commando, you might like the brute power of the Barbarian or the swift combos from the Monk. For those who prefer range and kiting, you might like to try out the Demon Hunter who has a wide array of traps, bows, arrows and dodge moves, or the Wizard and all the arcane powers that the class possesses. Finally, there's the interesting Witch Doctor class with the amusing African accent and his (or her, if you choose a female) ability to summon all sorts of insects, golems and other sorts of odd-world creatures to do his demon-slaying bidding. With all these classess, and extremely different playstyles, it is easy to see why Diablo III can be played over and over again.
Apart from mindlessly clicking your way through the game, Diablo's new direction on combat focuses a lot more on action-packed elements with a slightly modified control scheme – two skills on your left and right mouse buttons, and five other abilities in your hotbar of one to five. So you have about seven different skills at your disposal. These skills can be further developed as you increase in levels and unlock runes to change the way they behave. For example, the basic ability for a Barbarian's stomp ability is able to stun foes in a radius around him. You can modify it to draw all enemies near you or deal more damage on top of its basic stun by picking different runes (with different effect) of that skill.
They have also simplified the character building RPG-esque concept. Gone are the days of complicated skill trees and getting the perfect cookie cutter set of skills and abilities. In this simplifying attempt, most of your character growth will be based on the items you get from the loot as you explore the dangers beyond Tristram, or in the controversial Auction House that lets you use actual world currency, like the USD, as well as the in-game virtual gold currency.
The Auction House will be the one place you will visit frequently because most loot in Diablo III will not drop for your character level. It will almost always be lower in level than what you can use and will hamper your progress by a lot when you take on the harder game modes like Nightmare (I will talk about this soon enough). Unless you have a friend who is your gaming Santa Claus who can provide you with better loot because he is of a higher level than you, you may find yourself visiting the Auction House just to get a better weapon or armor.