Users can now leave comments on annotations, making them nice jumping-off points for focused discussions. Also, each "trail" gets its own Web page as a "trailhead." That's so Google will pick up the trails and index them.
Anyone can read Trailfire notes and follow trails without a browser add-on. You need the the Trailfire software to create trails, though.
I like the Trailfire (and Stickis) concept, and this one is nicely executed. Web annotation is an interesting form of hypertext (CEO John O'Halloran even dropped the obligatory Memex reference when he demonstrated it for me). I'm not sure the world is ready for yet another way to navigate the Web, but for particular uses--leading people on tours of various Web sites, mostly--Trailfire looks very handy. Follow my brief Web sticky note trail for a real-world example.