What is the advantage of using abstract classes instead of traits?

I can think of two differences

1. Abstract classes can have constructor parameters as well as type parameters. Traits can have only type parameters. There was some discussion that in future even traits can have constructor parameters
2. Abstract classes are fully interoperable with Java. You can call them from Java code without any wrappers. Traits are fully interoperable only if they do not contain any implementation code

For whatever it is worth, Odersky et al's Programming in Scala recommends that, when you doubt, you use traits. You can always change them into abstract classes later on if needed.

When extending an abstract class, this shows that the subclass is of a similar kind. This is not neccessarily the case when using traits, I think.

Abstract classes can contain behaviour - They can parameterized with constructor args (which traits can't) and represent a working entity. Traits instead just represent a single feature, an interface of one functionality.

1. A class can inherit from multiple traits but only one abstract class.
2. Abstract classes can have constructor parameters as well as type parameters. Traits can have only type parameters. For example, you can’t say trait t(i: Int) { }; the i parameter is illegal.
3. Abstract classes are fully interoperable with Java. You can call them from Java code without any wrappers. Traits are fully interoperable only if they do not contain any implementation code.