I agree that most problems with interfaces derive from the multiple inheritance workarounds rather than the interfaces themselves. But from a non-purist perspective they're generally quite nice to work with, particularly when you're designing a large system. And using copy-and-paste instead of MI is IME never an actual problem because you can either define it in a common base class or move it to another class and use composition.
I also agree that for most of the standard library interfaces are not used enough. There are exceptions (collections, io) but they prove the rule.
As a workaround (to a workaround I guess) there's a good deal of activity around different AOP solutions to implement mixins, even applying them to compiled classes. The only problem with this is the typical problem with AOP -- you don't know by looking at the implementation what it can do or how it can be used, you've got to have knowledge from somewhere else (configuration files or other classes). Or you have to rely on an IDE to do it for you, and then you're tied to a particular tool, which may be even worse.