When playing the game, READ! Especially the small usage tooltips at the bottom of every itemwindow.
It's one of those mistakes I've made myself, but now I'm smarter.
Many faqs I see here have to do with things that are constantly in your face. Blizzard really did a good job of adding those little tooltips and mouse-icons to items. We just need to read.
Thank you for this obvious, yet useful advice. I've been very pleased to note, when playing Titan Quest, that a lot of useful informations about the gameplay were regularly displayed as a tutorial on a side of the screen. I'm glad Blizzard did the same, under a form or another, in D3.
Blizzard also did a pretty good job of HIDING some absolutely essential information Like those advanced tooltips mentioned above; you'd have no idea that your spell damage scales with weapon DPS if you didn't turn those on.
The other essential hidden gem is elective mode. TURN IT ON in options. It lets you assign any available skill to any available button/hotkey, which you CANNOT DO without elective mode.