As is evident by posts such as the recent one from Gevlon, there is definitely a perception that sometime after Citadels are released, there will be a player effort to create a kind of alternative trade hub in high-sec. Any player group that is able to create, hold, and convince others to use their hub would begin reaping the benefit of the sales tax and broker fees. Those fees are a magnitude larger than most other income sources in EVE and thus have people reeling from their possible impact on the game.
But there's a "secret" when it comes to Citadels and it is two-fold:
First, and probably the most well known, is that Citadels will not have contracts on their initial release. This has come up in forum comments, the #structures channel, on reddit, and in other sources where details of the Citadels are being fleshed out. Citadels will be getting contracts in the near future, just not on the initial release. Contracts are a big part of any trade hub's operation and their absence will curtail all but the most dedicated traders from creating a truly large hub.
Second, and the lesser known, is that Citadels have limitations around ranged orders. If you are a miner, for example, and want to sell your product at the local Citadel because it is conveniently located, when you dock, you will not be able to sell to any buy order that does not originate in the Citadel. This is because the market system cannot account for a remote buyer's access to this Citadel to pick up your goods. Ranged buy orders from other Citadels or NPC stations will not show up as "green" in the market window when docked. While this presents an opportunity for Citadel owners or savvy buyers to set up orders in public Citadels, the initial reaction might be one of confusion from sellers looking to unload their materials. The reverse is also true. If you set up a Citadel with the hopes that you can use it to base your regional orders from and save on tax and fees, you can, as long as those orders are filled at NPC stations. Players docked in other Citadels and goods to sell cannot sell to you. They either have to move the materials to an NPC station to reach your order or, more likely, you will be cut off by a buy order created in the Citadel. Buyers are likely to need significantly more orders to manage all the public Citadels or miss out on those materials. This is not a bad set of features, per se. I believe, however, that it differs wildly from what the perceptions of the market feature is.
The truth about Citadels is that they are intended to be corporation and alliance markets. You will be able to open your Citadel for docking and for service use. You will collect fees from those players who choose to add to your corporation and alliance market or even their use of your reprocessing facilities. What you won't have is a true NPC station replacement that interacts with the "real" EVE market in the same way. You will be able to sell your own local wares fine but you will not be participating in the larger regional market when it comes to buying materials at range from other Citadels or in selling your materials to other buyers.
I believe that the lack of contracts on release will certainly delay any chance of a true market hub Citadel from appearing at first but it will be the idiosyncrasies of ranged orders that may, in the end, be enough of a problem where the true material sourcers will simply avoid Citadels in favor of NPC stations and where materials are sourced, so too the buyers will flock, at least in high-sec where public markets rule. The benefits for null-sec and wormhole corporations in running their own corporate markets are massive and there the adoption will likely be significantly higher.