All you need is common sense. This game is not hard. People need to stop having these delusions of grandeur about Diablo 3. For the love of God, Torchlight 2 is slightly harder game than this.
This like many other games are played by a wide range of gamers in terms of age, skills, time available, arpg experience and so forth. Just common sense and game is not hard, doesn't fit to everyone, some old players don't have the brain speed as the ones in their 20s, and kids and teeagers don't have the same knowledge of more mature and experienced players, probably the most successful gamers are somewhere in between.
If the game was easy and just common sense was enough, it wouldn't give any kind of challenge and everyone would have the same godly items, and probably bored after level 60, as it should be for any single player rpg, that's what is Diablo after all (but diablo is about item and get the char to the top, it's not LoL or gw2, either game genre or goals, and will never be imho).
Nothing is hard when you know it. The problem is that some people like you expect that everyone is super experienced, fast at understanding all game mechanics, with long nerdy past on diablo games.
Now, my point was totally different thing. If it wasn't direct enough, I meant that Diablo is a good game for what it's supposed to be in my eyes, the pole/carrot to be the best in DiabloProgress/Streaming is what keep the game alive up to now together with the subtle carrot of making some sort of money, whether you are interested in making money or not.
I play games casually to relax and study their business models and dynamics (both D3 and GW2) and my profile should be able to tell easily, the only aspects I'm really passionate about in successful online games are the development and psychology, in this case the role of real money in the present and future of online gaming more from a game developer perspective.