1. It doesn't work on Treos. That puts me out of loop right away.
2. I'm uneasy with all these context switching apps for mobile devices. I mentioned this before in the context of Google's (GOOG) Gmail applet, but the same criticism applies to Yahoo Go. I don't want to have to continually change apps to do stuff. I live in my browser, even on mobile, so make your tool work there.
So ... fix those two problems and you have yourself something. More seriously, this business of portable apps is a real dilemma. You can't do much interesting in current mobile browsers, but I really, really don't want to get into having to flip back and forth between myriad mobile apps. Please don't make me do it.