Chewing on the third metric, I'm looking at something that I'm going to call clarity but which I'm happy to find another name for if there are suggestions. it shows up in a few of the different points I mentioned, but the general idea of this: the GM is the stand in for the player's senses. She is their window to the world around them, and a lot of the game is going to depend on how well or poorly she does that.
The problem is that this is something that's best judged by the players. That's not a bad thing in its own right, but it's problematic for our purposes - we want the GM to be able to self-assess with at least a moderate sense of objectivity, so we need some metric to help the GM tell whether a given session has gone well or poorly, and I don't immediately see a good option.
The solution is to widen the net a little bit, and think about what we're looking at in general. We're trying to get a sense of how well the GM conveys the world. What does it look like when that fails?
Thinking as a player, this is really easy to point to - it's an undo situation. "Wait, what? I wouldn't have done X if I knew Y!", like "I wouldn't have tried out the window if I knew the ogre was right in front of it!"
Now, GMs handle these situations with differing degrees of grace, and I admit there's a danger that some GMs may not notice these situations, or may misattribute their cause, but calling it out like this hopefully helps any GM looking to rate herself. So let's call this our zero scenario.
What does it look like when the GM rocks at this? That's harder. Like a lot of good GMing, it's success is pretty seamless. The players had all the information they need to engage things, so it all just worked from their perspective. That's our 2 point scenario, but how do we spot it?
My suspicion is that you can tell the difference between a 1 and a 2 by the questions the players ask, specifically whether they ask for explanation versus clarification. Explanation (which usually sounds like "Hold on a second, [QUESTION]?") indicates that your first pass did not create a clear image for the players. Clarification is a question built upon the description - sort of a "tell me more about [THING]".
Admittedly, this is a bit of a cheat. We're indirectly using the players to judge this metric without explicitly asking them for a rating, but as noted at the beginning, they're the best source for this one. That part worries me less than the fact that this one may be a little bit harder for GMs to self-apply - it requires a decent recollection of the way the game went - and that may yet prove to be a real problem. Still, for the moment, I suggest.
0 - Player confusion regarding situation leads to complaints, retcons, arguments.
1 - Lack of clarity requires further explanations for players
2 - Sufficiently clear that questions focus on the situation as presented.
Honestly, this doesn't feel quite as solid as the last two metrics, but I think it's still in bounds. Still, I'm inclined to kick it a bit.