You need to install the plugins to get the most out of Quicksilver - for instance the Google search plugin. Also try the Flashlight interface which mimics the Spotlight interface but in the top left corner. There's also another one where QS works from within the menubar - you can't get much more low key than that.
Virtually anything you can do on your mac you can do through Quicksilver, including accessing menu bar options for applications, resizing pictures, mass selection and manipulation of files etc. You can browse your iTunes library, control iTunes and rate songs etc without opening an itunes window. You can look up words in the dictionary or Wikipedia, send emails, upload pictures to Flickr or even run Spotlight searches letting you search within the text of documents.
By default you can launch apps with a single keypress (e.g. holding down 'S' will load Safari for instance) or if that's not enough you can set up hot keys or hot corners for complex tasks (e.g. drag file to bottom left corner to attach to and send an email).
Quicksilver's functions aren't immediately apparent. It isn't just a launcher, more a new interface for your computer. To be honest QS is so good it's made me think that the traditional 20 year old mouse and icons interface seems incredibly slow and outdated.
Sorry, I'm a new convert - hence my evangelistic zeal.