In what way is this not just a giant null-op?
It is basically a lot of ado about nothing, with a grandiose sounding name which keeps on perpetuating the myth that the EU is imposing legislation (most legislation has to be approved unanimously, and the UK has veto rights), while doing nothing at all.
In two years time they'll actually start repelling some legislation, and that's what will be tricky, since every law protects/benefits a certain interest group. We'll see years of in-fighting about former EU law.
As a result, the UK does not have veto power and the EU is pretty much a legislative body without the appropriate representation.
They're changing the law permission from read-only to read/write.
It's not likely to be strictly symbolic, but on the symbolism front it has to keep May's government viable in the face of internal divisions within her party, and also able in principle to win an early election. Possibly more importantly, it cannot look like a repudiation of treaty obligations -- if the UK doesn't carefully follow the process outlined in the Treaty of the European Union for withdrawal, and doesn't follow the TEU and TFEU in the meanwhile, it will be difficult to persuade other parties to enter into future treaties with the UK. After all, break one crucial treaty, break them all...
Non-symbolically, it may try to set up some sort of review commission that will propose changes to the incorporated acquis; presumably this will be a body of, by and for the executive in the initial bill, unless they realize that in order to ease passage through Parliament it should include legislators (and possibly devolved bodies) from the start. It might plausibly go even further and propose that the executive be given outright secondary legislative authority on the argument that the executive is the "natural" replacement for the Council and Commission, and that Parliament simply doesn't have the resources to enact primary legislation fleetly enough. That is, the Cabinet will be able to make sweeping changes by fiat, with only subsequent "review" by Parliament.
It might go further still and touch on judicial matters -- for example, there are many questions about the disposition of matters before the European Court of Justice, and many of them are being raised by the hardcore hard Brexiteers who sit behind her in the House of Commons (and also some Remainers whose votes she will still need).
Apart from waiting for the text of the bill, we'll also have to wait for several matters before the courts -- not just the courts of England and Wales -- on the limits of executive power with respect to giving the notice under TEU article 50. It seems hard to imagine that there is in any statute a general power of the executive to strip real rights from people without primary legislation; such a finding would cause enormous conflicts with previous decisions in administrative law. It's also wholly unclear what the courts will make of the devolved governments' arguments that absent primary legislation to amend the devolutionary acts (these are basically the constitution of the federal system still called the UK) the executive can negotiate on matters that are strictly in the competence of the devolved legislatures.
Ultimately, this bill is directly comparable to a constitutional amendment, but with a scattered amending formula hiding behind the long-term principle that the amending formula is simply whatever the current Parliament supports. The reality is not that simple, and there is a crisis between the Tory Brexiteer faction(s) who want it to be exactly that simple and the Tory realipolitick faction(s) who admit that some matters are so entrenched that Parliament simply cannot unilaterally change them and at the same time argue that a plebiscite can supersede Parliament's authority (and thus Article 50 notice can proceed without reference back to Parliament). The courts, generally, will tend to side with the fiction of Parliamentary supremacy, so it is likely that the Great Repeal Bill will also have to give May the power to give Article 50 notice in the first place, unless a seperate Act of Parliament does so beforehand.
Finally, it is very strange that in her conference speech yesterday she announced that the Attorney General will personally represent the executive in at least one court case. In a way that's throwing Jeremy Wright under a bus, since the case in question is virtually certain to be a loss for the government, and he is likely to be accused of not doing enough because he's not a Brexiteer himself. I wonder if she will have to replace him, and whether the next Attorney General will be a Brexiteer.