For me it's been either Jaylen's improvement or Kemba's defence.
I always thought Jaylen would, at best, be a 16-18PPG guy with great defence, and solid rebounding. Instead he's a 20/7 guy with great defence and a TS% of about 60%. His handle has impressed.
I was under the impression Kemba was a poor defender. Sort of IT-like, in that he was simply limited. However, he's shown to be an excellent charge-card, can generate steals and pretty well holds his own.