@ mods, being fairly new here I'm not certain if this is in the right place, my apologies in advance.
Wall of text to follow - you have been warned!
So with the advent of Guild levels and guild perks in cata I believe there is a now a new way to print money: selling guild perks!
Rumor has it that some bigger, more established guilds have already started to ramp up for this, but I have not yet seen a wide proliferation of the concept so I figured I will post it here for discussion.
The business model in a nutshell:
1. Recruit people into your "guild" on the understanding that they are there solely for the guild perks associated with the guild levels.
2. Profit by collecting the extra 10% gold generated by the guild perk: Cash Flow
That's it.
Analysis:
The "target market" here is obviously not hardcore raiders, but rather people who prefer to play solo, in small groups, or people leveling alts that cannot gain entry into most serious raiding guilds until 85.
I believe that the appropriate way to market this is that it is not a guild, but a service that offers all the benefits of the guild perks. It would also be a free service, from the player's perspective, because the extra gold is generated by the system and not taken from the player.
The biggest hurdle, assuming that there is a fairly good supply of the type of players who would use this service, is the initial leveling of the guild itself.
To the best of my knowledge there isn't any hard data on, relatively, just how much it takes to level a guild. Early on the proposal would not be attractive to prospective "customers" because at level 1 you are not really offering any perks.
The easiest solution to this initial problem is to actually pay players to do things in your guild that will contribute to its level. The devil is in the detail of figuring out how to track that sort of activity and allocating the appropriate reward for it.
From your perspective you want to avoid free-riders, and also eliminate any perception of bias. Again we will have to wait for more solid data before these details can be worked out.
However I am also poor, and I most likely can't afford to find the initial capital to pay people to power level the guild (IPO anyone?

). So ideally there is some way to convince people to do this for you free of charge, most probably on the promise of being able to benefit from the perks once the guild reaches the appropriate level.
Related to this issue is that some guild perks, such as guild items, appears to require guild achievements, and not merely guild levels. Whilst these perks do not directly make you any money, they do make the service more attractive, and thus you will have more customers.
The beauty of this aspect is that these items require guild reputation to purchase, so you know people who are joining for these perks are guaranteed to make you money!
Again the cost benefit analysis is impossible to do with the current lack of data, but once you have maxed out the relevant guild levels for the passive perks, this will be the direction for further expansion.
When it is all set up, however, the guild levels and achievements form a natural barrier to entry against competitors, and it would not be hard to imagine that you would probably monopolise the market on smaller servers.
And, the beauty of it, after the initial work, is that it is passive income (the best sort of income), even if you are unsubbed!!