There are two immediate things that strike me as cool about Tafiti- first of all, its built in Silverlight and is a great showcase for how the Silverlight platform can enable much richer web experiences than we have seen before.
The second aspect is as a different angle on vertical search. Rather than doing a vertical focus of topic (like CouponLooker which specializes in coupon codes), it focuses on helping when you are using search as a research tool. I probably won’t be going to Tafiti if I just need to do a simple query to find the name of a web-site I need to visit, but for drilling down into a topic across book results, news, photos and more, it provides some neat capabilities to “bookshelf” results as you go along and come back to them.
All done in a gorgeous UI that should help clarify the sort of thing that the Jackson Fish folks mean when they talk about great software experiences. Congrats to Microsoft for pursuing this project and finding great ways to experiment with new innovative interfaces.