Conservative generally means using tried and tested technologies that has run stably for years on thousands of servers and where every corner case is well documented and well understood. Not using some new largely untested technology that sounds awesome on paper, but has yet to truly prove itself in the real world.

Disclaimer: I'm a huge fan of clojure and Hickey and I think Datomic sounds amazing. I'm currently looking for an excuse to play with Datomic and learn more about it, so don't take what I said as any indication that I'm somehow against Datomic.