Well...
1. When friends leave for another server (probably would have went with them).
2. When the staff members no longer care for players or the server.
3. When the server itself heads toward the wrong path (custom cards, donation items, etc that are too overbalanced).
4. When the server has no new updates for a long while.
5. When players start to bash and troll at you for no good reason.
6. When it is difficult to start with a new server as the community is not as helpful or that there is too much at once (hence I like smaller servers).
7. When there is a constant lag or downtime (foreshadowing the fate of the server).
8. When there is a huge community debate with no side giving in, and that the forums is messed up as a result as well.
9. When there is funny stuff behind the scenes, and players made a huge drama scene out of it all.
10. When I lose interest in the server and decide to go server-hopping.