Template:Skill development notes
From Diablo Wiki
All character skills underwent considerable changes during development, with perhaps the biggest change coming before BlizzCon 2010 -- skill trees were removed (but the tiers remained), and passive skills were separated out from the skill listing and renamed Traits. The second big change occurred when skill tiers were removed for the July 2011 Press Event, allowing skills to unlock steadily from levels 1 to 30; also, traits were renamed back to passives. Finally, the Beta Patch 13 introduced a new categorization system, which placed each skill into one of 6 (fairly arbitrary) new categories for each class.