Reflecting on Impedance Mismatches

I've been playing around a bit with MongoDB, and in particular its ReactiveMongo non-blocking asynchronous client for Scala. It's fitting in beautifully with what I'm trying to do with it (which is basically a standard CRUD webapp where the primary object in the domain model is some anonymous, arbitrary JSON block), even more so when I brought in the Play ReactiveMongo plugin.

Now I feel I'm really "living the dream"; I can concentrate entirely on my Scala code, sequencing in database fetches and stores when I want them, and not having to worry about the efficiency of table joins (never mind the tedium of getting them even working in the first place!) or mapping column types. The oft-mentioned "SQL-to-OO impedance mismatch" is gone, but even better, the ORM-to-useful code mismatch (which probably gives more pain in the long haul) is gone too.

Which got me to thinking - where are some other impedance mismatches in modern software development, and can they be eliminated as nicely?