There's a bit of a barrier to entry for Datomic because it really doesn't look quite like anything that you're familiar with if you're just used to SQL and NoSQL. The basic starting point is one big many-to-many relation, what in SQL might be one big table of (db_id, subject_id, verb_id, object_id, timestamp) of these "facts" which are (subject verb object) tuples, and these queries can be limited by the transaction timestamp. I'd say more about it but honestly, because it's closed-source, I've had real trouble figuring out what exactly is going on in the backend; a lot of the organization is done with namespaced keywords so I am not sure how exactly the datatypes are organized, but you can query based on subject, verb, or object quite rapidly. As Rich Hickey says, one instrumental thing is that these tuples are only inserted -- they are never updated or deleted. This enables a "many readers, one updater" architecture without violating consistency when you're updating the data: nobody uses a timestamp greater than their recorded one until the model (in the MVC sense) tells that view that it's ready to update. There is a mechanism for naming new elements in your transactions so that you can insert a bunch of facts at once.

It's also a little difficult because it is substantially meta -- instead of a special syntax ("CREATE TABLE" etc.) for handling structure, the information about how the database is organized is stored in the same "fact" tuples that make up the rest of the database, just with some automatic verbs that come bundled in. I'm always a fan of self-expressing systems, but that might throw some newcomers for a loop.

I guess my main comment about Datomic is, "I wish there were an open-source version out there." I'm interested in seeing what the plumbing of such a system looks like and perhaps learning something from it -- and I'd like to peek at their Datalog engine and so forth. Unfortunately, just from peeking inside the JAR, it appears that one might need a good understanding of Google Guava and Apache Ant and the Jetty server and a bunch of other things to be able to read the source, which might make it prohibitive to peek inside the code. I'm more interested in learning from the system than I am in using it.