I'm not a Metallica fan, but I find this small controversy interesting because I produce audio for 5 radio stations here in Nashville. (I don't produce music, mind you, but I produce commercials, PSAs and the like. Producing music is much more tedious and intricate, but still uses the same principles as what I do.)
I'm not going to compare the album to the Guitar Hero version simply because I don't have it on GH. But I am going to take an objective look at the waveform of the first track of DM "That was just your life" and compare it to And Justice For All's "One". It is a fair comparison to take one Metallica song and put it beside another because we're comparing the technical differences that translate to audible differences.
I took these screen-caps earlier today, and I will share my opinion with you regarding them.
You can see here that "That was just your life" (top) has a much less dynamic waveform than "One" (bottom) even in the most busy and heavy point in both songs.
Sometimes the timing of the song can make the waveform to have the appearance that it lacks dynamics when zoomed out.
Yet if we look closer you can see that even down to the second of music the first track on Death Magnetic has very little dynamics compared to the loudest and most heavy point in "One". What does this mean?
When you take a waveform and reduce the dynamic range and raise the levels, the tracks "run into each other" and start to sound like white noise. This can be done a couple of ways. One way would be "Hard Limiting" a track. Hard limiting (depending on how it's done) is easiest explained by saying it chops off any sound beyond a specific decibel level, usually you would raise the levels of the rest of the waveform closer to that level to allow all the different nuances to be louder. But if hard limiting isn't done sparingly and carefully, you end up destroying your louder sounds to bring up the rest of the sound.
Sorry about the wall of text.
I don't think this album was "Hard Limited" but I do think they reduced the dynamics another way so that the album just sounds "loud as hell". But even the average person without a trained ear can hear at certain points of the songs there is too much noise and not enough distinguishable sounds. Cymbols sound like loudly hissing snakes rather than smooth attack and sustain.
Basically what I'm saying is the album DOES sound like shit, and you can even see it on the waveforms.