Eddie Vedder and Chris Cornell's vocals, for the most part. The rest of their respective bands were just not good enough to carry them, IMO. They are just irritating to listen to.
I can see the similarities between Silversun and Smashing Pumpkins, but it is not a fair comparison. Silversun Pickups have incorporated the Pumpkins overdubbed wall of sound approach to their songs and share the same initials with the Pumpkins, but that is about where the similarities end.
I like the Silversun Pickups, and they are a pretty good band in their own right, but they have yet to produce an album that is on par (or nearly on par) with Siamese Dream.