I think there are two key differences:

1) H2G2 is full of entries written in the style of Douglas Adams by fans of The Hitchicker's Guide to the Galaxy trying to be informative and funny in that dry English style. It seems like everyone is trying impress The Man with their witty penmanship.

2) Everything2 is really a work of (and about) popular culture. H2G2 is more a reference piece - trying to be the lonely planet, encyclopaedia britannica and epinions all in one.