The "traditional" (and I don't know how you could call the latest version of any of the major databases "traditional", this is a brutally competitive market) databases don't lock just for the fun of it, but to enable features that users want. Anyone can come up with a product that doesn't do Y if it can't do X either. So what're we missing here?
I didn't follow the rest of your comment, I'm afraid -- I was just saying that I didn't see how using append-only storage immediately makes concurrency control a non-issue. The comments from the RethinkDB guys upthread support that: not supporting concurrent writers makes your concurrency control much more straightforward.
The "traditional" (and I don't know how you could call the latest version of any of the major databases "traditional", this is a brutally competitive market) databases don't lock just for the fun of it, but to enable features that users want. Anyone can come up with a product that doesn't do Y if it can't do X either. So what're we missing here?