Funcom's upcoming Age of Conan is the Guild City System. This is mostly unique to other MMOs, in that you build the guild city not as a hall, but as an actual city with multiple buildings within. Each building serves its own purpose for offense and defense against other guilds and attacking forces, and is upgradeable to different tiers, much like an RTS.
In a recent article at MMORPG.com, they run down the details of each of these guild city buildings, and their biggest advantages. Also in this article are some details on guild leadership itself. In order to create a guild city, your player will need to be a guild leader, level 40 or higher, and have a guild membership of at least 24 players. You will then have the choice of 3 environmental locations currently for your guild city. Let the land-grab begin!