For anyone who doesn't know Faker, his mechanics are sublime. Insanely quick reaction time and able to juggle multiple ability and buff cooldowns in his mind to pull off immensely complex plays and make them look effortless. There are plenty of videos and GIFs that I can post to demonstrate his skill (and I will link to the one someone referenced earlier in this thread,
Faker vs. Ryu),
but honestly if you don't play League or DOTA, it's just going to look like gibberish or just look like a dude killing another dude. It's understanding the nuance and mechanics behind the play that make it insanely impressive.
The reason it's a big deal that he's on Twitch now is that most of the Korean and Chinese pro teams have been locked in long-term exclusive streaming contracts with Asian services like Azubu or Afreeca that have large viewership overseas, but little to no exposure worldwide. Faker being on Twitch is going to do insane numbers, and he will likely make more from streaming than his already rather lucrative contract with SKT T1.