Takedowns on enemies grant permanent Attack Speed.
Dark Harvest is quickly becoming a prominent jungle keystone as it allows you to front-load a lot of damage on incoming ganks to help secure an easy kill.
Dashing with either [Q] or [W] makes for an easy way to get a quick 10 Lethality, giving Lee plenty of early game kill potential.
Zombie Ward is especially good on junglers as it allows you maintain high vision control throughout the entire map.
Lee Sin is all about his positioning and getting into the lane for a gank as quickly as he can. The extra Movement Speed from Relentless Hunter plays a big role in keeping you on the map and relevant everywhere.
Triumph's burst of bonus healing after a kill/assist allows you to get away with some risk tower-dives and survive so that you're able to snowball a lead to victory.
The extra Attack Speed from Alacrity is fantastic, especially in the early game, for empowering your jungle clear and passive auto-attacks to keep you healthy.