Nice comparison of the top mapping services over at TechCrunch, with lots of good additional discussion in the comments. As much as I love Google Maps, I do agree that the new beta version if Yahoo tends to be better, and that the helicopter bird's eye photos on the new Microsoft Maps are more useful than a satellite map in most real world cases. Sad that nobody mentioned the simple elegance of the old Mapblast Line Drive Directions, which I fear will become extinct once Windows Live comes out of beta.
As I said in my comment over there, the real killer app is true route customization. More than just shortest/fastest/avoid highways, but a true "send me this way" or "don't send me that way."
I'm imagining checkboxes or buttons next to each step along the route allowing you to eliminate and reroute accordingly, and maybe a "via" box to add a street or freeway that you want to take to push the application in the right direction. Integrating with live traffic data would be the next logical step, automating the rerouting based on live or historical traffic conditions.
Hear that BoRyan? It's our destiny calling...