In your restaurant example, I'd say there's no distinction between the second and the third option. Even though there is a dynamic market of different restaurants, by virtue of the fact that they are in competition with each other, they are homogeneous at a different level. Obviously not in the same way where all restaurants are collapsed into one enormous food court under centralized control, but in the sense that they all draw from the same pool of patrons, suppliers, employees, infrastructure, the same legal and monetary system, etc. This fact means that it is, in principle, always possible to game the system through those avenues. By virtue of the fact that there is a playing field, it can be tilted. The possibility for abuse of power still exists, although it's a different type of abuse than the arbitrary whims of the chef at the one giant restaurant in town. One important difference is that it's much more abstract and esoteric. It's easy to identify power when the chef refuses to cook eggs because he hates eggs and you have no other choices - this is a transparent kind of power. But when your favorite seafood restaurant goes out of business because market manipulations behind the scenes caused prices to suddenly spike, we don't see that as a result of power, it's even naturalized as the market at work. An important point is that at the naive, everyday level, everyone appears to have made a free choice - the restaurant owner, the patrons, the newly unemployed workers, etc. A concrete example of this is credit default swaps - some homeowners in California default on their loans, and suddenly people in Iceland have to pay double for food.

In the example of restaurant reviews, you could indeed create another competing aggregator if it had been corrupted, but then you would need some kind of aggregator aggregator, possibly a search engine. The example here is torrent sites - its a decentralized system, but for that to be usable, torrent search engines sprung up so that you could find things. You can't possibly track down and sue everyone seeding a torrent, but you can shut down the handful of search engines which is just as good. Further decentralization just creates the same problem at a different level.

File sharing is also a good example of what I'm talk about with the different kinds of power. If the cops raided your house because you borrowed some books from a friend, that would immediately appear to you as an expression of power. But the absence of a backup option when you put a DVD into a computer doesn't (OK, unless you are technically savvy). Notice how this is operative at the level of language: if you ask someone to backup a DVD, they will try and then tell you: "You can't do that." But as we know, you can do it, it's been disabled. Here again, power is naturalized, prohibitions and controls are thought of as inevitable or physical impossibilities.

My point is that we are concerned about the overt forms of power, and pay very little attention to the covert forms of power, those which determine our very perception of freedom vs. non-freedom. One of the effects of network decentralization is to make power more abstract, which is part of the reason we have so many conspiracy theories today. Conspiracy theorists are like creationists, they see complex phenomena emerge and attribute it to someone behind the scenes coordinating it and pulling all the strings, which puts a face on it, naively unabstracting it. A paranoid DRM conspiracy theorist might believe the MPAA sent operatives into his house and installed software that disabled DVD backups. There's some kind of twisted truth in it, but the problem is that his mode of resistance is all wrong. Now he thinks he can just hole up, board up the windows, buy a gun and he will be safe.

So this is another reason why developers are important - they can see and resist power without naively unabstracting it.