I suggest each player has a counter for how many times they "leave" a mission before its complete.
Once it reaches a set amount, there are consiquences.
But you can see the counter in your Heroe page (but not amend it), and see how much your pushing your luck.
The idea being that everytime you leave a mission before its completion, it wil be recorded by Anet somehow.
This is then related back to your information page.
Once the counter reaches (for instance) 10, 20 or a higher-value, you will be punished.
Lets suggest that once you reach;
(10) You are banned from forming human parties and get 10% less drops for a week.
(20) You are banned from forming human parties and get 20% less drops for a week.
(30) You are banned from forming human parties and get 30% less drops for a weeks.
etc etc etc until
(100) You are banned from forming human parties and get 100% less drop for a week.
You might also, or instead of that, get a "forced title" onto yourself such as "(10) Leaver - dont accept me" for the span of the week. The number changing depending on the values above.
Once the week is up, the title vanishes.
Once you reach 100 and that week ends, the counter resets to 0.
Before I get comments along the lines of
"But people get disconnected at times and kicked out of the game".
To try and avoid misunderstandings, the game would watch you.
Once you leave a mission, if your still in the game and active inside the game, then it knows your a "leaver".
If you left the mission and logged out, or your inactive after you leave then it can assume you got disconnected or your sessions froze or lagged stupidly.
I know this isnt perfect and im sure the numbers and punishments will get debated, but you get the idea.
f
