I think engagement is about as good a metric of success as I can think of. When you see players deep into roleplaying their characters, cheering as they roll a critical hit or sweating as they roll badly to avoid dangerous consequences, seeing their sense of wonder grow as they discover and interact with something new to them in the world, that's all them showing that they are engaged and care about the game.So yeah, fun. Engagement. But you want deeper than that so what does that mean to me?
It is interesting that a few people have talked specifically about "accomplish"ing things at the table. that isn't really a thing i think about too much. That is probably at least partially due to me being an improv, pantser GM and not usually having any sort of goal structure other than what the players have created for themselves.Did we accomplish something meaningful content wise?
Absolutely. We may not have reached our goal, but we should at least be closer to it, or have a few avenues closed off at least. That can still feel like accomplishment under the right conditions.I think the only reason I have a thoughtful answer to this is because of a discussion from another thread:
A successful session for me means I understood what the goals were, I was able to move forward toward those goals without wandering around blindly, and along the way I had the opportunity to make decisions that would move us toward our goal if successful, but would set us back if unsuccessful.
(Note that it doesn't require that we were actually successful in moving toward that goal....)
Most of the time when we don't accomplish as much as I had hoped when I did my prep for the session, it's because we were having too much fun getting sidetracked with something the players found interesting and decided to follow to see where it went. It's often not something I had considered digging into further, but they found it interesting and had fun interacting or exploring which usually means I did a good job making things up as we go along. The only time I have ever stopped a session and admitted "that doesn't lead anywhere important" is for playtest one shots.It is interesting that a few people have talked specifically about "accomplish"ing things at the table. that isn't really a thing i think about too much. That is probably at least partially due to me being an improv, pantser GM and not usually having any sort of goal structure other than what the players have created for themselves.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.