I like to describe Java interfaces as "multiple inheritance from abstract base classes that live in a secondary type system." Here are four drawbacks:
- They're a secondary type system completely separate from the primary type system in declaration and use. Is your object a foo or does it merely implement a foo? Why should there be a distinction?
- They don't provide any code (unless you want to talk about constants defined in interfaces, for which Java's rules have always confused me).
- You can't add them after the fact to a class without editing its source code and recompiling. This probably springs from the first problem. (If they'd solved the first problem, you wouldn't need to do this!)
- The standard library doesn't take them seriously enough. If you're going to declare String as final, at least give me some way of providing something that works like a string that the standard library can accept. I think this is also part of the first problem above.
The first problem is the most damning from the purist's perspective. Java's designers recognized a real problem (a single-rooted inheritance scheme is inadequate to express real solutions), but they created a secondary type system and didn't take it seriously.
From a practical perspective, the second problem is worse. The designers recognized that a single-rooted inheritance scheme is inadequate to express complex behaviors of real problems, but they completely failed to allow for any code reuse! "Objects that implement this interface all behave similarly, but multiple inheritance is bad so you'll have to copy and paste code to make it work."