Agree vertical is a bit suck. It's because it's much more complex to navigate - you're twice as likely to get lost as your brain works hard to remember it's position in a 2D space, much harder than a 1D "line" I think.

The need for drill-down is valid, but I some means of entering/exiting a new context would require less brain power. Like tabbed browsing. Or push/pop from a stack.

We're trying to crack these problems with some presenation tools we're prototyping. Fun fun...