Senior Game Designer Jeremy Feasel (he has an NPC named after him, the ultimate honor) has been answering questions on twitter about garrisons. Quite a few, in fact. Under his twitter handle of @Muffinus he dished out some pretty interesting details on what the current design for the feature is.
- Garrisons are going to give you perks outside of them (we already knew about the faster mount speed perk, but others are being worked on).
- Some perks, like a free resurrection, will not carry over into raids so as to keep them from becoming mandatory perks everyone has to get for their garrison. They want to keep customization as a draw and let players pick the perks they want.
- You aren't hard gated by the initial quest that establishes the garrison - you can pursue the benefits and perks of the feature even if you decide to bypass it when leveling.
- You will not have to stop doing something to run back and defend your garrison if/when it comes under attack. It's not a time-limited mechanic.