Bets on Unohana being permadead. She's about to get some sudden development in the form of a flashback, and it's largely the same setup as Gin.
As for her "sin," I'm guessing that's related to Yachiru Kusajishi being Unohana's daughter (or whatever the equivalent is for soul society). Unohana's original name being Yachiru is likely more than coincidence. I also might be crediting KUbo too much, but when Kusajishi was original found by Kenpachi, she was completely unfazed by the blood and violence she witnessed, which would make sense if she had been around a murderous Unohana. Kubo has connected plot points from way back before, so it's possible.
Kenpachi's weakness is a bit harder to discern. Perhaps it has something to do with him limiting himself, in that by trying to enjoy the fight he's not trying his best to win, and therefore is weaker. I'm guessing half his power up is going to come from Unohana slaying him multiple times, proccing his saiyan nature, and the other half is going to be his overcoming the mentality that has made him weaker since his first encounter against Unohana.
Edited, Jan 23rd 2013 4:55pm by Allegory