Personally, i don't like the idea of Guild Halls, for me, an MMORPG need a ton of people in your screen, but if you add some system like housing or guild hall, you decrease the population, mainly of the main cities, giving the impression of a dead game, where you don't have the high level characters in the main city, only new players.
For me, guilds are just a system, where you press a button, and then appear a screen showing details of your guild, like his level, banner, members etc.
And about levels, i think that systems where you need to upgrade you guild to have access to new buffs is a great idea, but, those buffs need to be something more focused on utility than effects for your character, for example
Guild level 1 - Nothing
Guild level 5 - More member space
Guild level 10 - Caravan Pet (allows you to summon a pet where you can change your transmog)
Guild level 15 - Decreased cooldown of summoning guild members
Guilde level 20 - Upgrade the Caravan Pet (Now he has a guild bank and a personal bank)
And on and on
But things like gold buffs, an really easy way to craft things or get items (Like an pet who is an Auction House or a blacksmith for example), i think those take out the importancy of the big cities and having to plan your journey through the world of the game.
PS: XP buff i don't care that much.
+10