Where to start? I could go into how Rawls makes a big mistake in what the people would decide when he claims that 'freedom of religion' would end up a human right. It can't for elementary reasons and they wouldn't anyways because many atheists wouldn't stand for it. That's his liberal prejudices at work, much like Kant's Christian prejudices led him to claim that suicide was immoral. I could even go into how Rawls makes a fundamental, and monumental, mistake in his definition of a 'minimal being' since he describes a database without any motivation (ie, not an agent). But that would be trifling with details.
The big problem with Rawls' A Theory Of Justice is this ... it's impenetrable. If you can pick up a programming language specification and inhale it all in one sitting, then yeah you'll do fine. Otherwise? Forget it. Rawls' actual theory could be written up in 40 pages but his book has 500 pages. Of course its impenetrability is common to all philosophy texts. They're all padded with historico-linguistic shite because philosophers confuse the history and the terminology of their field with its actual subject matter.
This is similar to how physicists screw up physics education by mashing mathematics and history into actual physics. So anyone who's tried to learn about quantum physics will understand what's missing in A Theory Of Justice by comparing mainstream physics textbooks with Scott Aaronson's essays on the subject. Essays like Quantum Computing for High School Students and PHYS771 Lecture 9: Quantum.
But this impenetrability serves to conceal a much deeper and intractable problem with the book. A Theory Of Justice is nothing but a work of propaganda targeted at moral philosophers. It's written in such a way as to ensure that the smarter you are, the less chance there is you'll call bullshit on it. It hammers you over the head with references and tires you out with long-winded explanations. But it's bullshit. Not the "conclusions" Rawls comes to, the universal human rights regime predated Rawls by several decades, but every single step used to get there! A Theory Of Justice fails in its goal and is particularly annoying while doing so.
Why is it annoying? Well let's start with Rawls' attempt to pull a Galileo. He leads the reader down a winding road constantly saying "you know this, you know this, you've always known this" and you end up in a completely different place from where you started and the truth is *no* you did not know any of this shite which is the exact opposite of everything you ever believed. Propaganda can be convincing without being logical but when it aspires to be a work of philosophy then that's fairly damning. Manipulation is, or at least should be, a big no-no in philosophy.
What Rawls Should Have Done
Rawls starts with an individual(istic) human and tries to persuade and cajole this person to accept a collective perspective. But this is utter nonsense and the reason why is because morality is collective by definition. So there, just short-circuit all of that nonsense and start off from the collective viewpoint like any other theorem in mathematics.
MORALITY: blah blah collective blah blah
Because all you have to do to justify that particular definition of morality is to contrast it with a very similar definition of ethics. The only significant difference between them is that ethics is individualist and morality is collectivist. Ethics defines a being's (individual or group) relation to other beings that are fundamentally different from it. Morality defines the INTRA-relations of a group of beings who are fundamentally similar, since they're all part of the same group. So all you have to do is set up two definitions, compare and contrast, and bingo you've got collectivity. By definition. And that takes care of a good couple chapters of Rawls' book.
Once you're at this collective point, by simple definition, all you have to do is go 'lo and observe', this is freaking mathematics. You've got a definition, right? Now let's start with our assumptions. What are the assumptions? Let's start with ... nothing. And bingo, you've got something very close to the veil of ignorance. You don't need to go on a long-winded rant about how allowing people to use knowledge of their station in life to determine their station in life is circular reasoning. Which is a weak argument anyways. You don't need to make that argument because it's obvious: any theorem you can construct using the fewest assumptions (knowledge) possible is automatically stronger than a theorem using more assumptions (more knowledge).
And at this point you introduce the analogue of self-consistency and contradiction. And of course you're talking about a meta-structure of morals. Not one morals but a whole set of morals (theories) dependent on what kind of knowledge (assumptions) you made to begin with. And you can reason about this meta-structure.
You can observe that this structure has minima everywhere, infinitely many of them, but only a small number of maxima. So obviously the maxima are more important. And for all you know, there's only one global maximum and at that point it becomes extremely important. And this is all automatic due to familiarity with mathematics. The things that are rare are those that matter more.
And so it "happens" that a society with the most extensive system of human rights possible is a just society. Not because we pulled it out of our ass like Rawls does but because this is what we label the maximum because we really care about that maximum and not because we have any preconceptions about what "justice" is.
Actually, we do have a preconception what justice is, it's what we would expect should happen. But how does this connect with Rawls' notion of "the most extensive human rights"? Oh right, because we're supposed to find these rights in our self-interest, because Rawls has written a propaganda book. Not because ... oh real people in a society must choose a notion of morality and mathematics tells us there's only one special case that stands out. Because hey, that would smack of inevitability and not self-interest. And we all want to keep self-interest in morality, right?
And that's Rawls' work in a nutshell. An attempt by a liberal to rationalize a communist idea, universal human rights, based on egotistic self-interest. Horribly misguided and boring too!