http://motls.blogspot.com/2007/11/exceptionally-simple-theory-of.html

The author is not constrained by any old "conventions" and simply adds Grassmann fields together with ordinary numbers i.e. bosons with fermions, one-forms with spinors and scalars, neglecting any traces of dimensional analysis, too. He is just so skillful that he can add up not only apples and oranges but also fields of all kinds you could ever think of. Every high school senior excited about physics should be able to see that the paper is just a long sequence of childish misunderstandings. I understood these things when I was 14.

There is not a glimpse of physics in that paper. You won't find anything like a "Lagrangian", "amplitudes", "masses", "cross section", "quantum corrections", "anomalies", "energy", "force", "Hamiltonian", "entropy", "phase transition", "path integral", "renormalization", "temperature", or other words that you expect in a particle physics paper.

Of course, the comments that this theory may be tested by future experiments are nothing else than journalists' confusions. The theory can't reproduce even basic aspects of the observations that have been made decades or centuries ago which is why any talk about even more ambitious tests are rationally unjustifiable.

Also, if you have ever heard about "GUT" (Grand Unified Theory), you should realize that unlike E6 and others, E8 cannot be a grand unified group because it only has real representations which is not good enough to create chiral fermions. In string theory, E8 is only relevant because it is broken to a smaller subgroup by intrinsically stringy effects. Needless to say, Garrett Lisi has no idea what a "chiral fermion" or even "fermion" means so he is not worried about any of these "details".