It's a really fun game. I was in closed beta for a long time. I was waiting for open beta because they aren't doing anymore character wipes.
It's a "Diablo" game, but there truly are different aspects to the game. First, there aren't skills like a traditional diablo game. You use skill gems that are socketed into items that you use. Then there are support gems that support the skill(s) you have socketed into the items you use. For example, you can get a bad a.ss socketed weapon where you put a FIREBALL gem in there, but then add FASTER PROJECTILES and CHAIN (I forget the exact name) support gems. What this does is turn your normal fireball into a fireball that shoots much faster AND will chain (think chain lightning for D2). That's just one example of many different combinations that people use.
The other vastly different part is the skill tree. Instead of traditional skill points (or lack there of in D3) you get a skill tree. People call it a spider web or whatever, but basically, there's 100s of points that you can invest into to make your character the way you want him to be. There are things like faster attack speed, more hp, more mana, ect... but then there are KEYSTONES where you gain special abilities. One that I liked (it just got nerfed when it went to OB) was Chaos Innoculation. It made you immune to Chaos damage (a damage in the game that pierces your armor and sh*t), set your health to 1, gave you an extra 50% energy shield (just like a life pool, but it's called energy shield) and made your health work off of energy shield. It was great for a lot of builds because you could stack energy shield and make some OP characters. They nerfed it though, so I don't think it'll be as good.
The items in the game are really cool as well. The uniques are good. The rares are very good as well. They have a really cool crafting system that allows you to reroll items and make them better. They do not have a currency (like gold) but instead they use the crafting items as a currency. There are orbs like the Regal Orb that are worth quite a bit. There's another one called the Mirror or something like that (I never found one) that allows you to duplicate an item. They are rare and Godly to where you can literally get anything in the game with it. They added new things called Maps that are more end-game sh*t to where you can open this Map and go to higher leveled areas to find loot. They actually come with little things like "Armor is half effective and double frost damage"... basically, the harder they sound, the better the loot you can find.
Overall, a very fun game. I'm going to play it again once I fix my computer. It's different enough to peak your interest if you're looking for a 'diablo' game and are fed up with D3.