What are the fundamental problems of being in a world with only 2 spatial dimensions?

We don't know all the laws of physics for 2d. Hawking's argument, mentioned in this answer, may seem plausible if we insist to use the same blueprints of our world to a 2D one. But even so, there are living beings whose digestive system doesn't prevent them for being simple connected. They are coelenterata and cnidaria, and have only one opening, both for "input" and for "output":

Other argument, raised by Eddington, and reiterated by Tegmark, is that the atoms and the solar systems would not be stable, because gravity and electric forces would no longer vary with the inverse square of the distance. This, again, if we want to apply the same rules. This argument is a bit surprising as coming from Tegmark, who made it in the context of his mathematical universe hypothesis. He argued that any possible universe, described by any possible mathematical structure, exists, but we live in one which allows for intelligent life, and unstable orbits would not allow. If we consider all possible worlds, why consider that the Poisson equation should be valid in all?