Easy to use API's that are very well documented so you can get up and running quickly without having to read through tons of code examples.
You have access to all classes in DB4O which is great when working across multiple platforms/environments (Windows / Linux). There could be more documentation around some features - it would help if there were tutorials written specifically about how certain methods work! We're using this library within our iOS mobile app where we need an easy way to persist data between sessions while also allowing us to easily switch databases at runtime should one go down.