User blog comment:Xilinoc/Ch. 545 - Blue Stripes/@comment-4822693-20130709222211/@comment-4822693-20130710214438

First off I wasn't talking about any theoretical ability of Ichigo's reiatsu to kill people. I was talking more about 1) his bankai, and 2) the fact that even captains and lieutenants need to wear special seals on their power when they go to the world of the living. Think about the problems for the SS if every time Ichigo used his bankai they had to travel to the world of the living and recreate a couple of mountains. SS has a lot of powerful kido and technology, but even they probably can't just "put a mountain back" if Ichigo destroys it with his bankai.

I think what Shunsui is saying is that Ichigo is parting ways, not permanently, but rather he just won't be around very much anymore. He won't count as a human, so he doesn't belong in the world of the living for extended periods of time.