SavageBob says:
There's too much work that goes into putting a game on for others to not have some way for us to forestall disruptive players early on.
The main problem I see with this whole discussion even is this:
-> As has been stated, the players
most likely to just up and disappear without any warning are players that are new to the site
-> As players that are new to the site, they won't have the posting history to have any sort of metric system indicate anything about their possible disappearance
->Therefore, the best possible metric available to GMs to determine a player's potential is one we already have: The date the player joined GamersPlane. Anything beyond that - creativity, writing style and ability, or anything else I can think of - can't really be judged via "metrics" anyway. We'd essentially be back where we started - there wouldn't be any way to determine whether folks that are new to the site are "in it for the long haul".
ADDENDUM: It usually becomes pretty clear who any "problem players" (hate to even really use that term, though they admittedly exist...) are by the time you finish character generation. Trust your gut and address any issues that come up up front with honesty and maturity.
I've never had a player who has spent a significant time on this site drop out of a game of mine without some notice of warning first. (Except for one, but he was extremely active until he suddenly wasn't and he went silent across the whole site.)
And an anecdote.
The first game I ran on GP was a disaster. The simplest one-shot you could imagine, and we didn't even come close to the end of it. All the players who joined throughout were new to GP. Over the course of barely a few months, we had I think five or six different people come and go without a word - only one, maybe 2 of the original party members were still involved when we finally took mercy on it and put the game to rest. The one who made it to the "end" is still very active on GP. The others vanished, never to be heard from again.
This experience taught me a LOT about what makes a PbP game hum, as compared to a real-life game. One of the most valuable lessons I learned from that is to run material that gives you plenty of escape hatches as a GM to deal with 1) people suddenly disappearing and 2) introducing new PCs to replace them.