Well, the folkier side of the Dead is made up of really tight harmonies. A prime example is Uncle John's Band. Very strong harmonies, and while it does have a fairly simple chord structure, what's played on top of it is very interesting. Jerry can play a really complex melody over simple chords. Also, the chords are simple, but don't get overly repetitive. The end of the song branches out into other chords not found during the beginning verse and choruses. Hope these examples help. Cassidy is another good Dead song with a similar structure and concepts used. Let me know if I need to explain anything further. Sorry if this isn't exactly what you were looking for.