Editor's built into the game,
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In Mod mode you can hit TAB to switch between editing the level + playing it.
These two points I find very interesting. In a good way.
A Very Good way.
I love the idea of being able to rapidly change between playing/creating. Something i've often wished for in EVN, and SketchFighter less so.
Oh, would it be possible to get around the Directory crap by storing the mods in the same folder (Or a subfolder thereof) as the game itself? I don't know much about windows, and certainly nothing about the directory, but... Oh, i dunno. You guys just do whatever's easiest/best.
Man, do I wish EV Nova had the capability to test the damn mods without having to quit ResEdit/MissionComputer, load them into the EV Plugins folder, and then fire up Nova, only to find out that your projectile has the wrong flags on it, etc.
So yeah, integrated mod editor = win. That's what made editing LodeRunner (the Mad Monk's Revenge, in particular) so fun - if you wanted to test out timings on disappearing blocks or teleporter links or something, you could just hit the "test" button.
I also agree with Xander about Library/ApplicationSupport; as a longtime Mac user, I can tell you that it will make a lot more sense once you start playing around with it. Also, I would recommend putting a link/alias to the mods folder in the main application directory, just for ease of access. And if you want noobs to stay out of the game files, just encyrypt them like IV did with Uplink and Darwinia, but please, don't put them entirely outside of player access. Making the editing system cryptic is much better than inaccessible. Plus, it gives us wannabe hackers something to take a whack at.