Filter all the DB by the type of documents they are most suited for. Find out what kind of querying is available - maybe you need just access by ID or you plan

Filter all the DB by the type of documents they are most suited for. Find out what kind of querying is available - maybe you need just access by ID or you plan to write full blown queries doing complex selects. Indexing - what kind of indexing is available (if required)? Is it IDs only, or you can index any attribute in you document? That's my main points. After that: speed, scaling and replication (not int his order).