Developing a Gaming Vocabulary
We lack terms to discuss a lot of what goes on during games. We have lots of terms for things that are within the rules, Hit Points or THAC0 or Skill Ranks, but relatively few terms to describe concepts that are not covered by the rules.
We have terms for genre (high fantasy, western, space opera), but we lack terms for types of games beyond a basic of axis of combat vs roleplaying (aka is it hack-n-slash or not). A game can be episodic or regional, center around traveling and encounters or it can keep delving in the same locale. It could be all about plot and NPCs or just treat other characters as background.
These aren't monumental differences, but having terms for them would help GMs (and players) recognize what the differences are, letting them consciously make choices and sculpt their game. I tend to use comparisons to other games instead (“it was an Escape from Ulshadore scenario” or “it's a a West Marches style game”) but that only works if you played in that other game — not exactly a portable term.
As far as the participants go, we know there's a GM (or DM), and we know the other people at the table are all Players. If we were old school, we'd even say one player was the party Caller.
But we lack terms for the interaction between the people at the table, for particular behaviors that come up in games again and again. The player who can't figure out why their character would want to go along on the adventure and holds up the whole game struggling for motivation (“I'm a druid, I just can't see myself going into the city”). The player who tries to guess the entire plot in advance when there's still hardly any evidence and sends the whole game into left field (“No, there weren't any fingerprints on the gun because he must have been an alien shapechanger sent here by an intergalactic overlord from the future!”). Players that congratulate themselves on predicting obvious tropes of the genre (“I'll bet these villagers are endangered by some monster and want us to kill it, ho hum”).
Most of those are negative examples, but there's the positive too. The player who intentionally throws a bone to involve other players (“Gee I don't understand it, but I'll bet Doc Carter could figure it out!”). The player who metagames ignorance of hazards and gets their character into fun trouble (“Zombies? That's crazy talk! I'm going to the cemetery right now with just this flashlight.”).
Defining terms helps us recognize what we are already doing, and maybe do the good things more and the bad things less. It's easy for a GM to recognize that including a Power 15 monster in a Power 2 game is trouble, because the concept of a creature's danger has been codified as Power. Wouldn't it be nice if the GM had the same terms for other things to watch out for that could spoil the game? (q.v. Question Your Assumptions)
The odd thing is that without a vocabulary it's hard to even recognize what things we don't have terms for. I suspect that as we keep building this language more and more things that need names will be revealed.