Well, like I said Slick is not an ORM whereas Diesel is. Slick is not as much about an ORM abstraction, but writing SQL queries in Scala by leveraging it's collections library, taking advantage of Scala's advanced features such as operator overloading, etc.

For instance this is an example from Diesel:

Post::belonging_to(user).load(&connection)

And this is how it would look like in Slick:

Post.filter(_.userID === 1).result

So both of the above would translate into SQL "SELECT * FROM posts WHERE user_id = 1", however Slick is more explicit whereas Diesel definitely feels like an ORM. The tripple equals in Slick example for instance is a nice touch and an example of operator overloading, because userID could be of type Integer or Option[Integer] (nullable field) and it would work both ways.