I mostly bought my Mac mini for Front Row, and while that hasn’t disappointed me (especially since Apple added the ability to shuffle music by playlist), the real killer app has turned out to be something different:
The ability to play almost any music video, movie trailer, comedy clip etc through my TV has proved to be a really great feature. If friends are round and something comes up in conversation, such as the new Portal Trailer, I can have it playing on the big screen within about 30 seconds, with no need to hand round a laptop, or get anybody off the sofa.
I have to say I totally agree with you.
I watched something awesome on Jools the other day, and normally I would keep it on my Sky+ box and show people. Now I just send out a YouTube link.
Also – the fact that youtube has almost every 80s music video, just makes it an absolute killer app!