I've tried to make myself love Alfred, and it's admittedly very polished for what it is, but to me it seems to miss the entire thing that makes Quicksilver just so productive to use: the [object][verb] syntax.
Alfred kind of bizarrely stops half-way there: you type anything to find the object you want to manipulate, and then... pick a verb by hitting the appropriate command key or arrowing through a list. My hands are already in the middle of the keyboard, just let me type the verb name!
I think its fair to say quicksilver is more powerful, you can really do almost anything in it. I however didn't use many of the power features (resizing images or composing e-mails).
Alfred is really just a launcher with a few nice bonus features (clipboard history & spell are my favourite) where as quicksilver is more like a command line power house.
Alfred kind of bizarrely stops half-way there: you type anything to find the object you want to manipulate, and then... pick a verb by hitting the appropriate command key or arrowing through a list. My hands are already in the middle of the keyboard, just let me type the verb name!