Turn-around time is what matters most. (I work on a console game). You need very fast turn-around time for a designer to test his ideas. If something turns slow, later it can be put in a C/C++ function and exported.

Also sand-boxing, but this is not of concern to the actual scriptures, it's to the rest of the team.

And something to easy deal with tasks/threads, or state machines, and such.

Also a language that allows you to declare easy what needs to be kept persistent (goes in the game save), what can go through the network (game-play related feature affecting all in multiplayer), or what can be local "effect".