The reason I, and probably others, find landscape reading so important is that I have a lot of reading materials (academic papers) in PDF, which don't get reflowed when zooming. Once you've zoomed to the point that a single line doesn't fit on the screen, it becomes basically impossible to use. This would happen with landscape mode, too, but you can fit much more on the line

But your comment about annotations being properly anchored makes me realize that positioning annotations is not as trivial as it seems, particularly when the annotations go along with text that is being reformatted to fit the screen. That actually helps to explain why landscape mode is difficult, since it adds one more layer to that problem. It would not surprise me at all if these problems are connected in such a way that fixing one fixes the other. I guess we'll find out-- I'm looking forward to getting even more use out of the Edge with landscape mode.