You need to have a way to keep players playing on your server. And the best way to do that, is have goals. There needs to be an objective, otherwise the game gets stale, and no one will want to stay. Whether it be having a ranking system, where you purchase ranks with in-game currency or missions / challenges for the players to complete for a small reward. But while at the same time, not making it too difficult for the player, otherwise they may not want to even try. And definitely don't give "donators" an overwhelming advantage over free players.
