If I know or see someone to be a really good GM, I feel confident in the depth of the material for the story, and their ability to filter out characters that might make the story less realistic (e.g. overpowered characters). When other participants show some care in trying to understand the world the GM has created, and are respective of other characters, that's quite exciting as well. That's how I generally judge whether an RP will be exciting and successful - at least for me personally.