Over time, Quicksilver learns about your choices and makes them increasingly probable as matches. You might have to type an entire word the first time before it goes to the top of your list. The second time, just typing the first letter make your item the very first choice. Over time, all things you commonly use are only a couple of keystrokes away. In other words, Quicksilver "learns" (man, that's a stupid thing to say).
Because items are indexed by natural names, you don't actually have to remember much of anything. Most old-school shortcut programs force you to remember cryptic key combinations, whereas Quicksilver requires only knowing the name. Using Quicksilver is about as natural as these sorts of programs get.
Quicksilver is not the only application to take this approach, but it's so good that I don't care to investigate anything else. Now, the project is open source, which likely means a serious escalation in quality, features, and undoubtedly a long and productive life. It can even appear on inferior operating systems and make them suck less. Through its spread, it might even permanently change the way people expect to interact with computers.