Dbrane, aside from "beauty", the electroweak unification is actually needed for a finite theory of weak interactions. The need for all the fields found in the electroweak theory may be explained step by step, requiring the "tree unitarity".
This is explained e.g. in this book by Jiří Hořejší:
http://www.amazon.com/dp/9810218575/
Google books:
http://books.google.com/books?id=MnNaGd7OtlIC&printsec=frontcover&hl=cs#v=onepage&q&f=false
The sketch of the algorithm is as follows:
Beta-decay changes the neutron to a proton, electron, and an antineutrino; or a down-quark to an up-quark, electron, and an anti-neutrino. This requires a direct four-fermion interaction, originally sketched by Fermi in the 1930s, and improved - including the right vector indices and gamma matrices - by Gell-Mann and Feynman in the 1960s.
However, this 4-fermion interaction is immediately in trouble. It's non-renormalizable. You may see the problem by noticing that the tree-level probability instantly exceeds 100% when the energies of the four interacting fermions go above hundreds of GeV or so.
The only way to fix it is to regulate the theory at higher energies, and the only consistent way to regulate a contact interaction is to explain it as an exchange of another particle. The only right particle that can be exchanged to match basic experimental tests is a vector boson. Well, they could also exchange a massive scalar but that's not what Nature chose for the weak interactions.
So there has to be a massive gauge boson, the W boson.
One finds out inconsistency in other processes, and has to include the Z-bosons as well. One also has to add the partner quarks and leptons - to complete the doublets - otherwise there are problems with other processes (probabilities of interactions, calculated at the tree level, exceed 100 percent). It goes on and on.
At the end, one studies the scattering of two longitudinally polarized W-bosons at high energies, and again, it surpasses 100 percent. The only way to subtract the unwanted term is to add new diagrams where the W-bosons exchange a Higgs boson. That's how one completes the Standard Model, including the Higgs sector. Of course, the final result is physically equivalent to one that assumes the "beautiful" electroweak gauge symmetry to start with.
It's a matter of taste which approach is more fundamental and more logical. But it's certainly true that the form of the Standard Model isn't justified just by aesthetic criteria; it can be justified by the need for it to be consistent, too.
By the way, 3 generations of quarks are needed for CP-violation - if this were needed. There's not much other explanation why there are 3 generations. However, the form of the generations is tightly constrained, too - by anomalies. For example, a Standard Model with quarks and no leptons, or vice versa, would also be inconsistent (it would suffer from gauge anomalies).
The four quantum field theories (QCD, QED, QFD, and EWT) unite quantum mechanics and special relativity. They are all fully understood, complete, and proven.
In your quote for the standard model, there are not four distinct field theories, the electroweak has united the electromagnetic and the weak in one field theory, the electroweak theory. The standard model unites the strong QCD field theory all in one field theoretical formulation with the electroweak, three forces, electromagnetic, weak and strong. Ideally these forces at some high energies are unified into one force, and that is the aim of the GUT .
A Grand Unified Theory (GUT) would combine QED and EWT.
Quantum Gravity (QG) would unite QM and GR.
Quantum gravity is the quantization of gravity. Unite is not a good verb.
A Theory of Everything (TOE) would unite QG and GUT.
unify, not unite . otherwise OK
Loop Quantum Gravity is a proposed QG, but has not been proven*.
and has problems with special relativity.
String Theory is a proposed TOE but has not been proven*.
*correct, though proven is a bad verb for a physics model. Validated is the correct term. Physics models are accepted if validated by all known data and falsified even by one disagreement with data, when the model has to go back to the drawing board.
Best Answer
We already can do this in materials--- it is called "superconductivity". The phenomenon for photons was understood before the phenomenon in the weak interactions, and the description of superconductivity by Landau, and the Bardeen Cooper Schriefer model for fermionic paired condensates, was the inspiration for Nambu's fermion vacuum condensate idea, and Brout and Englert's later point-particle superconducting Higgs mechanism.
Photons do not go slower in a superconductor, they do not go at all. Superconductors don't have photon excitations at all, and if you have an electric and magnetic field in the superconductor trying to propagate, the fields decay away exponentially.
It is certain that we won't be able to do this in vacuum, because we know all the fields around us are stable. In order to make an instability in the field, we have to alter the fundamental constants in such a way that a charged field makes a superconducting condensate. In order to alter the constants, we would need a certain energy density per unit volume which is going to be practically infinite for the purposes of engineering.
But the condensed matter analog, the superconductor, is a perfect analog, and we understand the dynamics of what would happen in this situation simply by examining what happens in a superconductor, and extrapolating to the situation where the material doesn't break Einstein's relativity invariance with respect to constant motion.