Oh I completely disagree about the Spotlight menu being as fast as Quicksilver for launching apps but even besides that Quicksilver can do so much more. You can: set global shortcuts for damn near anything you'd want to do; add a line to a text file (for quick note taking); send a quick email; move files...

And for the Quicktime+Perian vs VLC thing, Quicktime will be decent enough for xvid or divx (though you'll take a small performance hit for it), but if you use MKV you will definitely want to use VLC -- Quicktime doesn't give you the option of switching languages or embeded subtitles, and the performance is awful.

But I do agree with the Transmission recommendation, it's really a great BT client: clean, not nearly as bloated as Azureus. Unless you really need some of Azureus' more advanced functions, you'll find Transmission can cover everything you want.