Ever tried dealing with vertical column text (e.g. Japanese?). I haven't myself, but the fact Microsoft made fonts on Japanese versions of Windows have special "@"-prefixed versions rotated 90 degrees, so if a document is written in it, then changed to that font and rotated, it's the right way up, makes me suspect it's pretty difficult.
That sounds like the result of a lack of forethought when designing the font rendering code, and not because it's a big problem. I've used vertical Japanese text input extensively in Word without any problems.
