In general, the sound is just carelessly mixed and poor sounding compared to the original mix. It's severely lacking in treble, with a low droning bass noise, and a muffled compressed sound. There is no dynamic range.
To quote Hairy Hen, the mixes problems include,
flattened dynamics, lack of high frequency response, poor balance between background and foreground elements, music cues too low in level or missing altogether, certain effects and dialogue being enormously distorted, incorrect placement of elements within the sound field relative to their position on screen, some effects even being out of synch to the image altogether
Here is a direct comparison of the 2004 mix (it's almost identical to the 2011 mix, outside of some small fixes) with Hairy_Hen's 70mm recreation (use headphones): http://www.mediafire.com/?avv1d8izol8hy3t
(Note that they did sort of "fix" the lack of music during the trench run in the 2011 mix, but the other examples should be the same between mixes.)
Notice how the 70mm has far more punch and dynamic range, while the DVD/Bluray mix has a sort of a droning bass noise. You can even hear clipping and distortion when the Death Star blows up in the DVD/Bluray mix.
Edit: found Hairy_Hen's detailed description of what is wrong on the 2004/2011 mixes: http://originaltrilogy.com/forum/topic.cfm/Star-Wars-1977-70mm-sound-mix-recreation-stereo-and-51-versions-now-available/post/548812/#TopicPost548812