I played through normal, and found it mostly boring with a few highlights. I tried playing a new class, but playing normal a second time was just mind-numbing.
Since everyone says "normal isn't the real game, you have to play the higher difficulties," I played through all of Nightmare. I found it even more boring the second time through. Even though there was actually a possibility of dying this time, it wasn't really what I consider a challenge. I still obliterated everything on the screen in seconds, but if I got lazy or ran into a group of enemies with a bad combination of effects I would die in a couple of hits.
Now that I've beaten Nightmare, I just can't even bring myself to start Hell. I'm sure its more "difficult", but not in an interesting way, just in the fact that enemies do more damage and have more HP. There's not really a new experience to be had there really.
In order for this game to be worth playing again, the following things need to change:
-Nightmare should be the default difficulty. Blah blah blah Nightmare starts at 30 so you can't... whatever. Normal mode has no reason to exist, and it drags down the entire experience having to go through it before getting to the "real game". Just get rid of it. Or, barring that, allow a player to skip normal difficulty on a new character if they have beaten Normal at least once.
-Higher difficulties should have more interesting aspects to them. Just adjusting some numbers and randomizing the dungeons doesn't really make it worth playing through the game a second time, much less a third or fourth. I can't imagine trying to beat Inferno with every class in the game; I would go crazy. There are games out there I can play over and over, and feel fresh every time, but Diablo 3 is not one of them.
-Do something more interesting with the loot. I want pieces of loot to encourage unique playstyles. I'm sick of finding a new piece of gear and just saying "Does that have more INT/VIT than my current item? Nope, vendor trash." Obviously not every piece of loot will be amazing, I just want more in the way of sidegrades. More items that might have lower stats, but produce unique effects that make them worth using (on hit effects that occur 1.5% of the time are not worth it really). More items that make you think "Well this isn't as good as my current equipment for my build, but if I switch to this skill/rune it could be really useful." Also, it would be nice if there was any difference at all between swords, axes, daggers, maces, etc. besides base attack speed.
Obviously there are more aspects that could be improved on. But those three are the ones that are sucking all of the fun out of the game right now, and if they were fixed it would actually be worth playing again for me.