So i went through the first act, i haven't played the beta so this is a fresh experience, and i must say it was fairly entertaining. At some point i had lag, which is annoying when playing solo, but it was on my end, i have high ping sometimes (probably because of one of my roomates).
It wasn't really challenging, as everyone said, except a couple tense situations with elite mobs but i never died (playing as wizard).
The way you have to choose spells by slot is interesting, it forces you to find what works best for you, without ever allowing the possibility of being gimped because you chose the wrong skills/stats. One could think that it's "dumbed down" but i would disagree, i think it's actually an interesting design. You still get to specialize your stats, anyway, but through gear, which can be switched.
I haven't used the blacksmith so far, since all my gear is already better, but i trained him anyway, i had nothing better to spend my money on (npc merchants seem useless so far, i never even needed item repairs yet).
Can't wait to unlock more spells to play with, i think i'll go back to it now, i just took a break after Act1 but i'm ready for more
Oh and just a quick footnote, i also tried Path of the exile recently, which got me bored almost instantly. I was pretty hyped up for both but only Diablo3 lived up to it. Though to be fair, Path of the exile is still in beta, but i wouldn't hold my breath.