Based on his one and only MMA fight in K-1, a little bit:

Based on this video, I'd say we know the following:

1. His striking is god awful. He has good power (I think), but this video clearly shows his rudimentary footwork and awkward guard position. People don't realize it, but the stand-up game is incredibly complicated and takes a long time to master. 2+ years isn't enough, not even for an athlete of Lesnar's caliber.
2. He does look to pass the guard. This is a good sign for Lesnar. He's not content to sit in guard. He actively works for better positioning once the fight hits the floor, a tactic that could easily overwhelm Mir if he's not careful.
3. His wrestling is fantastic. We already knew this, but his reflexes in catching Kim's kick and immediate knee-tap takedown are impressive.
4. He is aggressive from the outset. Expect Lesnar to close the distance on Mir very quickly and look to put him away with every punch or pass.

Beyond this, it's hard to say anything else. We don't know about his gas tank and we don't know about his submission defense. I'm sure he's working at break neck speed to bring those up to par, but until we've had a chance to evaluate them, they are unknowns. But all that and more could change Saturday.