One aspect of micro-benchmarks that is most of the time ignored is that they reveal different default settings various systems come with. And unfortunately after seeing the results, not too many dig deeper to figure them out.
Indeed. And the default settings reveal something about a system's priorities. If you care about your users' data, then Mongo's default settings should be a giant red flag. The revealed priority is seeming fast.
That was actually one of the explicit design goals of Rethink -- pick defaults such that users never have to wonder about the safety of their data. I know the folks at Riak are also in this camp, so there are definitely NoSQL dbs that do this well.
How about "they should be an invitation to learn more about the system" :D? As far as I can tell, in many cases these defaults have almost never been reviewed by devs because most of the early users already knew what tweaks they needed.
I dunno. Personally I'm a big fan of the "sane default or no default" approach. I think that mongo's former approach is irresponsible engineering (I understand they have fixed it recently).
(If your performance numbers are too good to be true, they might not be true.)