Biggest things for me would be, in no particular order:
- Mac support
- Physics that scale up with new machines. So.. multithreaded physics engine?
- Craft construction that allows circular connections of components (without the use of a zillion struts or auto-struts).
Other things I'd like to see, but aren't showstoppers for me:
- Integrated life support requirements (even if maybe it's optional.. Difficulty level based?)
- A "wind tunnel" to be able to test and display things like Center of Drag and Lift in different orientations, different speeds, different pressures, etc etc.. Basically.. Less trial and error building, more in game tools to let you test before you put it on the runway/launchpad.
Overall, really looking forward to this game!