Of course, if you are using Quicksilver, you don't need this. I wish there was a decent Windows analogue that I could use out here in corporate territory. No, make that I wish some enlightened soul allowed us to buy PowerBooks.
On a first glance, I really didn't like the plugin-oriented approach (or rather, I didn't like having to go and download plugins for core functionality that used to be there before). I think base functionality (i.e., a core subset of plugins that cover what you can do with a freshly-installed Mac) should always be part of a default install, and that things like Firefox and developer-oriented plugins should be part of separate "packs" or categories.
I'll survive the change, of course (and things like the diff plugin promise to become staple actions on my machines), but for a while it felt like it started out on that long winding road simple applications take in order to eventually become too complicated to manage - especially if I have to bother with irritants like managing identical sets of plugins on both my Macs.
Konfabulator springs to mind, and if you've been reading this for a while, you know that's not necessarily a good thing - openness and third-party contributions are excellent, but an unmanageable menagerie of add-ons becomes a hindrance to users and too distracting to developers (who spend more time adding stuff to categories than doing actual development)
Still, the new iTunes plugin rocks, and Nicholas has (as usual) been quick on the uptake and open to suggestions. He also pointed me to Growl, which might be just the thing for me to use instead of (or alongside) my homegrown RendezvousAlerter. Kudos to him :)