The big feature being added in the Warlords of Draenor expansion is the ability to create your own little city. A form of player housing, if you will, but instanced in a way that you don't need to compete with other players for land. They took a baby step in this direction with the farm in the last expansion and from the reading I've been doing today it seems like they've gone all in on making these garrisons awesome.
From the sounds of things you ultimately get to have 12 buildings and 20 followers in your little town. But there are 25 possible buildings and 291 possible followers so you sure don't get to have all of the things at once! So there's a lot of picking and choosing to be had as you build the buildings you most want to have and recruit the right followers to properly use those buildings. The buildings can all level up to level 3, and the followers can level up to 100 (and then can level up their gear) so there's an awful lot of numbers to make bigger in these things. I like making numbers bigger!
I decided to take a read through what the buildings do to see if I could come up with a preliminary plan for what I'll want to work towards. You get 5 buildings guaranteed (town hall, mine, herb garden, fishing shack, and pet house) and the rest are divided up by size of the building. You get 3 small buildings, 2 medium buildings, and 2 large buildings of your choice.
There are 10 possible small buildings and they mostly have to deal with crafting professions. 8 of the buildings are specific to a crafting profession (like blacksmithing) and are used to get the recipes for those professions, to give you access to more of the daily cooldown generated crafting materials, and to allow people who don't have those professions to craft most of the stuff. Like, for example, Darkmoon Faire cards, which can be crafted with the scribe house. That sure does make me tempted to get a scribe house for both of my level 90 characters... And makes me wish I had more characters to level up to get more scribe houses...
The other two small buildings are the salvage yard and the storehouse. The salvage yard looks like it lets you accumulate transmog sets (may be of interest to some people but not to me... I have axes on my head and shoulders!) and small amounts of gear for you and your followers. The storehouse lets you access your bank and the guild bank from your garrison and lets you start more work orders in the other profession buildings. It's unclear if that will increase throughput or just let you go longer without logging in to add more orders to the queue.
The medium buildings all do a wide variety of things. There's a pvp house which gives you titles and pvp related quests. There's an inn which gives you daily pve dungeon quests and gives you more control on what followers you can recruit. There's a lumber mill to generate extra garrison supplies. There's a trading post that lets you sell stuff for garrison supplies, gives you access to the auction house, and gives you a reputation gain buff. And there's a barn which gives you leather, meat, and a new crafting reagent used in all epic item recipes. It seems odd that everyone gets free ore and herbs but you need to use a medium building to get free leather. Sucks to be a leatherworker I guess? I also have no idea how hard it will be to get garrison supplies so I don't know if the buildings that give you more are awesome or just for people who don't play much.
There are 5 large buildings that do a variety of powerful things. The armory makes all quests you do have an increased chance of awarding bonus loot, lets you generate gear for your followers, and gives you free bonus rolls on raid loot. (You get at most 3 per week, but have to pay a fair amount of currency to get them. This gives you one for free.) The barracks lets you level your followers up faster, lets you fight in the open world with one of your combat followers helping, and raises the follower cap to 25. The mage tower lets you move quickly around the continent without needing to use flightpaths. The gearworks gives you a bunch of crazy engineering abilities including the ability to drive around in the open world with a tank of some kind. The stables gives you access to new mounts, lets you mine and pick herbs while mounted, and increases your mounted movement speed by 20%.
It's hard to know just by reading about these things how important the different things are going to be. Is the bottleneck going to be garrison resources? Follower levels? Follower gear? Specific followers? I believe you earn resources and followers just by playing the game and questing so my gut feeling to start is that they aren't terribly important for me if I'm going to be playing a fair bit at the start. So I probably don't need to worry about the buildings that pump those out? This makes me think my eventual setup is going to be a blacksmithing house, a scribe house, and a storehouse for small buildings. A barn and an inn for medium buildings. And a stable and a gearworks for large buildings. And I'm pretty happy the mine, the fishing shack, and the pet house are locked in!