Like some have said, I think an RP is 'good' if the players enjoy it. If the players/GM aren't having fun, then it's a 'bad' RP. I'm more used to table top gaming. That can be an 'art form' but in most cases, unless you were there, you will never witness that piece of art. Our written RPs have the benefit for being written, so people can read them after the fact. 

I wouldn't compare an RP to a 'play' but I would compare it to 'improve'. In the play, the actors have a script and they know what should be coming and how they should respond. But in an RP or improve, you really have no idea what the person will say or how their character will react. Then you have to come up with your own character's reaction to the changing situation. Since we are playing by post, a player has some time to decide how to reply. We can think about it before replying. Depending on the posting rate, we may have a few minutes to a few days to decide how to reply. But on stage, somebody doing improve has only seconds to reply. Improve is usually comedy. Can you imagine trying to do drama as improve? Well that's what our RP's are...sort of.  

What the hell, I'll continue talking. I love to hear myself talk or see my own writing.  :)
[list][*] Is RPing 'art'?
It can be. And maybe bits and pieces of an RP are art, but maybe not the whole thing. Again, it's the eye of the beholder. When I write a RP, I'm writing for the other players/GM in the game, not a wider audience. 

[*] If so, is it less 'art' than traditional media - say, [i]The Great Gatsby[/i]?
I don't know. Is painting more or less art than movies? Some RP can be good. But I personally judge a game by if I'm having fun or not.

[*] If one instinctively looks at one game and considers it 'better' than another, what does that mean, and what is that based on?
I guess for me it's on technical aspect of the writing. I don't mind spelling or grammar errors, if I can still understand what the player is trying to say. But if it takes me a long time to decode what the player was trying to say, then that's a huge negative for me. But other factors I look at are can I follow the story? Or is it so disjointed from post to post that it seems like nobody is responding to what the others have written. Are the characters believable? And for me, are they likable? 

[*] At what point, if any, can a game be judged? When it's being born, while it's still being played, while it's dead, or never?
As players we judge RP's all the time. When a new RP starts, we judge it to decide if we want to join or not. Once the game get's going, we judge it if we want to stay in the game, are we having fun? Or do we feel the need to leave the game, for whatever reason. But I guess the basic reason is that the game isn't fun for us. Or there are other things we rather be doing.  Yes, I know sometimes people also quit games because of RL issues, but that's another matter. 

Other than praise, I don't think anybody really wants feedback on their games. Unless asked, I would never tell a GM or player what they could 'do better'. 


[*] Do the 'Free', 'Casual', and 'Advanced' Tags factor in here? Even if not the sections themselves, which are unregulated, but the principles behind them?
This is a personal issue for me. But I hate Advanced RP. I don't want to have to read 30 minutes of posts before typing up my post. Just trying to keep up up on what's going on in advanced is something I don't enjoy. Again everybody is different and people like different things. I'm liking the shorter, to the point post. Maybe a few longer posts from time to time and when everybody is making their first one or two posts, introducing their character. But after that, seeing a wall of text that I need to read to before I post just turns me off. [/list]