I've been using the zen setting for a couple weeks now and am enjoying it for the most part. One thing I noticed is that I don't have access to the mute/ignore stuff in people's profiles, which I would like to still have access to so I can still regulate my feed.
If it were up to me, I would also like to still be able to view people's bios, although I can see why some would want that excluded in zen mode.
And one last thought: (although I could easily do this with a line of custom css), I wouldn't mind if the "sort by" line in feed, global, and chatter were to be removed in zen mode. Simplify the interface to just use feed, chatter, and global, without the number of shares, since it's being hidden on each individual post anyways.
I'd love to hear other peoples' thoughts on this, especially if they're using zen mode. I'd be curious to know how many others are using the setting.