It's different to D1 and D2. There are no real skill trees to speak of, just passive, offensive and defensive skills that you can alter with 'Runes.' All of these unlock at different levels. So at level sixty your character has access to all the skills. The secret is how you configure those skills with your equipment for optimal DPS / armour / HP and resistances.
So, technically, all characters are equal. Once you've built one level sixty barbarian there's no real need to build another --- because you'd still have access to the main skills. I suppose most non-addicted players might get two, maybe even three different classes maxed out before (a) they get bored (b) beat inferno in a group and / or (c ) the XP / DLC comes out.