Using Alfred for half a year, I finally found something not so convenient of it. Take "Activity Monitor.app" provided with OS X Mountain Lion as example.
If I type "act" in Spotlight and Alfred, both get to it with ease.
If I type "mon", though, Alfred cannot figure it out while Spotlight makes it.
Another example, also from system utility "Disk Utility.app".
If I type "disk", both Spotlight and Alfred get it.
If I type "di u", only Spotlight gets it.
Final example, from "DOSBox.app" and "VirtualBox.app"
If I type "dos", both Spotlight and Alfred get "DOSBox.app" only.
If I type "box", both get "VirtualBox.app" and put it first, but only Spotlight gets "DOSBox.app"
Maybe I am too strict on this since the main power of Alfred is Powerpack.