I'm level 50 now and working my way through Act 5 still, side-quests keep distracting me. I am feeling that something is a little off with this game. The map is huge but feels a bit like it's just full of nothing. I just use my horse (before that just dodged all the time) to spring past all the mobs to get to a quest waypoint, do what I need to do there and then rinse repeat. The side-quests and dungeons are pretty repetitive too, in fact so are most of the mobs. It's not too hard either regardless of the creep scaling (not a fan of this mechanic), I'm on WT2 playing as an Ice Sorcerer. The loot I'm getting is awful, some things I haven't upgraded in ages, the whispering chests rarely even drop armour / weapons for me, not sure why anyone would waste money on keys when the items you can buy directly seem to have a decent chance of being a legendary.
My memory of Diablo 3 is a little hazy but I don't ever recall thinking the same things about that. I'll keep playing for now as I am mostly enjoying it but not sure for how long or if I'll even bother with an alt. The fact you can only have 2 on local co-op means I won't getting any extra play time by playing with my 2 boys either.