Just recently, I've noticed an interesting difference between GMs 5 and 6. Game Maker version 5 has 18 room transition styles, whereas Game Maker 6 only has 14. The four that are missing are 'Push from (top,bottom,left,right)'. This is interesting. I noticed this when running an old Zelda engine I have, the rooms have a transition style identical to that of Zelda. The only issue with the transition was that the HUD was visible in the 'to' room, and the 'from' room during the transition, though this could be solved by making the HUD temporarily invisible.
What I wanna know, is why Mark removed this transition and whether or not it would be possible to make a script for it, a-la Da-Huntha's 'draw_text_sprite()' substitute script.