I don't know much about RethinkDB yet, but I will say that I have been a big fan (online) of one of its founders, Slava Akhmechet, for years. I've never met him, but he wrote some terrific articles on his website, http://www.defmacro.org/ , a few years ago. Start at the bottom of the list of articles, with "The Nature of Lisp."
Slava is a deep thinker, which makes me very excited to take a look at RethinkDB.
Indeed - he mentions in the article that he set himself a goal to convert 10 programmers into Lispers. Sounds like he probably has that many just in this thread! Kudos, sir!
He didn't make me a Lisper (I'm more a Haskell fan these days), but reading Slava's Lisp articles years ago was a significant part of what set me on my current career path.
He helped get me into functional programming, which got me a contract job [1], which is how I met one of my current co-founders.
I've joined RethinkDB just a couple of months ago, so I might not have all the historical facts right, but here is what I know.
In a previous incarnation RethinkDB was a highly optimized storage engine for SSDs implemented in C++ to be able to take full advantage of both low level SSD and kernel access.
The current distributed engine was built on top on this storage engine and I think it only made sense to continue with C++.
Originally, RethinkDB was to be a MySQL storage engine, which made C++ the natural choice.
They pivoted away from MySQL after my short stint in the beginning so I can't speak to why the storage code was kept (though I can't imagine it's because my code was so great they couldn't bear to throw it away).
Storage people tend to stick close to the metal, in general. This means C or C++ in most cases, for better or for worse.
Slava is a deep thinker, which makes me very excited to take a look at RethinkDB.