Object orientated productivity
QuickSilver is what it's creator, Blacktree Inc., call "An evolving framework for accessing and manipulating many forms of personal data." ... to me its a huge step forward in the way computers should work, and I can't wait to see how they extend it's functionality in future, especially with Tiger.
It addresses a flaw I've long found in computers which is an emphasis on application and operating system, rather than an emphasis on documents and tasks which seems much more practical.
To this end one extension I could see being great for QuickSilver - though maybe this is the programmer/CLI geek in me - is being able to access the script facilities of programs directly. So, for example, you could call up iTunes and then have access to all of the simple functions in the AppleScript Dictionary for that album that could operate on a third parameter. In a way this would give a shell/CLI like interface to windowed applications.
Having written that I'm guessing that it is probably me that would want it. It's also a bit like something I'd like to see in a shell, which is a way of simplifying parameter completion, which would be pretty hard to do in a shell context, but would work in the visual context that QuickSilver provides.