I don't think Spielberg is over-rated at all, I thoroughly enjoy his movies, and they have become darker over the years.
Lucas, yes I would say is over-rated, though frankly it's the acting in the Star Wars films, rather than the direction which lets them down.
Tarantino is not over-rated. He's made no secret of stealing ideas and scenes from Asian movies and the Hong Kong scene, so slagging him off for it won't change a thing. I think it takes a lot of talent to crib from other films, yet still produce a piece which feels totally original (though his scriptwriting could do with a bit of work - From Dusk til Dawn contains a lot of the same lines as Pulp Fiction, and probably True Romance, but it's been a while since I've seen it).
I'd say Lars von Triers is over-rated, definitely, but I don't think Kubrick is. Orson Welles, maybe - but he did do things with a camera which had never been done before, and which are used in abundance today. He may have been a great man, but I'm not convinced about him being a great director.