Simple: Its stats are twice as affected by modifiers (Swords Dance raises 4 ATK stages instead of 2, Tail Whip lowers 2 stages instead of 1, etc).
Klutz: Can't use held items nor the move Fling.
Unaware: During damage calculation, ignores foe's stat changes except SPEED.
Fluffy: Damage from contact moves is halved. Damage from Fire-type moves is doubled.