Wandering up to a tent to hear someone you never heard of is one of my favorite things about Bonnaroo. Last year it was the Dodo's that I discovered, along with The Gossip and Dr. Dog.
This year I discovered Band of Skulls on Thursday. Great set and a band to look out for to be sure. The Knux were also pretty entertaining. My biggest discovery this year however was how awesome MMJ really are on stage. I've tried to get into their studio stuff for a few years now and, unfortunately, had not seen them play live (I skipped 2008 and they weren't on my radar in 2006). So I can't claim them as a "new band" discovery, but damn, I haven't been so enthralled with a band in a very long time.