Passive skills.  They make gameplay fun, exciting, rewarding, especially in Softcore.  Damage skills, damage absorption, initiative, elemental protection, additional HP & MP.  Item and meat-drop skills.  Familiar level.  Crafting, Dual-wielding, other misc. skills.  Uses and combinations, depending on gameplay style.  Hardcore Ascension contest.  What skills to keep, in what order?  Begz0ring for questions.