EPTDiablo and WoW have a very important difference when talking about casuals. In WoW you have to get to max level and do some dungeons and raids in order to experience all of the content. With Diablo you really just have to get through the game on normal. And, in fact, this is what most casual players are going to do. They will play through the game on normal and perhaps follow up on nightmare. I'd say there is a better chance of a casual player playing through normal with every character than for them to get through inferno. Average Joe and his gf are going to beat the game on normal difficulty. This he is going to quit and his gf will keep playing until halfway through nightmare before she gets bored and they both move on to something else.
You are around level 30 when you finish normal. Nightmare and Hell gets you to 60. I think that almost all the player want to level to 60 esp all the people who played WoW. So no they cant make that hard. You will prob get some title like in diablo 2 and that will make even the casuals try to get through inferno too in my opinion.
And how hard can you make content rly that is soloable for a group of players ?
Hardcore is of course another story.
Diablo isnt a game where you leave the instance
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