How would it be like?
Few things to consider:
No need to state the obvious. We know lots of triggers mostly means good. Try to keep the most obvious to bigger points, like for having lots of triggers: "Having a power editor with a lot of triggers, but making sure to have an UI where I don't get confused by too many conditions/effects".
Picture modern game. Don't talk about tile layers, 2D graphics, etc.
Think realistic for a commercial game, don't go overboard with features, "OMG MAKE IT PULL OFF AN MMO!"
Profit
Anyway, here are mine, they're all up for discussion if you find them good or bad:
Interface! I believe one of the most underrated things in editors are the interface. If I were a tool designer I'd try to unload most of the things not being used in the editor (if in-game). This way, all the controls will be nice and responsive. Trying for an OS look also is good as we're all used to that interface. Putting things that are used often out front and things that aren't so much on the back.
Act like you're playing the game still. One thing I love about Age editors is how they feel like you're playing a game editing. Objects/units all act as if in game, everything seems like it's just being dropped into that world you're creating. There's no lines going off units to denote Z/X/Y-Axis and all those things that make editors look professional. Don't go all out for power, keeping editors simple while leaving all the fancy pro stuff as option can be a good way to do this.
If it's not working, please don't leave it for us to find out. This is a major kill for Age editors, the recent ones. Most of the people who tried going in the editor right after release ended up giving up editing, while a few stuck around to go after all the fixed until finally the new generation avoided all the bugs. Bugs can seriously kill the wish to design.
Customizability! People don't all use their tools the same way. Allowing people to edit the UI, make custom triggers (like in Age games) and being able to just hide away what they don't use increases productivity and make designing a lot easier.
Few things to consider:
Anyway, here are mine, they're all up for discussion if you find them good or bad: