One of the biggest productivity boosters in QuickSilver is the ability to use Triggers. Triggers can be, well, triggered from key combinations, mouse gestures (if you have the Abracadabra plugin loaded), or mouse clicks (with the Mouse Triggers plugin loaded).
If you have the User Interface Access plugin loaded, and I recommend doing so, you'll be able to use any Menu item from any application as the trigger action. This means, for one thing, that you could create new shortcuts within any application, simply by programming them in with QuickSilver.
Here's an example of what I did today: In Flash MX, I wanted to create several new blank keyframes, but this was only accessible from the menu - no keyboard shortcut! So I created a trigger that I invoke by clicking twice along the left edge of my screen (Mouse Trigger plugin). This triggers the Menu Bar Item (User Interface Access plugin) action "Insert>Blank Keyframe". Two quick clicks as I draw, and there it is: a new blank keyframe. Maybe there's another way to do this specific action, but this is an example of QuickSilver's flexibility.