That being said, one big part of prog's appeal for me is the whimsical and fantastical nature of it both musically and lyrically.
Some lyricists like Hammill, Waters and Wilson can go beyond the fantasy stuff and write some really profound lyrics but most of the time I like that prog bands have an impenetrable lyrical style, the lyrics may not have anything terribly important to say, but that's because the focus is on the music.
Most prog even "sounds" whimisical and fantastical on a music level, so if the lyrics were about the same things that people like Dylan and Costello write about, it wouldn't be a good fit. Prog has an out of this world sound and thus works best with out of this word subject matter. The fantastical, the mythological, the abstract and the surreal. Often that works better in prog than stuff about more social issues like what the punk and American folk movements were more concerned with.
I see nothing wrong with such "immature" subject matter, after all it's always the punk guys who say rock n roll needs a little immaturity amirite?
Take for example how Peart works better with geeky sci fi stuff than with corny Ayn Rand lite logician rants. Prog wouldn't fare as well with standard rock n roll lyrics either, like how Sinfield works best with lyrics about crimson kings, moonchildren, circuses and ladies of the dancing water and isn't so great with lyrics about getting "A taste of my love".

Most prog lyrics are cryptic as all hell but weither it's for a laugh or meant to be taken seriously, it suits the music. Weither or not guys like Ian Anderson, Jon Anderson or Peter Gabriel are full of crap, the cryptic nature of the lyrics add a mystique to the music and makes you wonder. That's kinda the beauty of prog, it f*cks with your head. If you want "straightforward" music, prog isn't the place to look.

Prog lyrics don't really need to have anything to do with "reality" since it could be argued that the music itself doesn't, so much prog has it's roots in classical music and lets not forget that classical music very often had romantic and fantastical themes rather than social ones.
People have been giving Jon a hard time on this thread, while I agree that he's no Dylan, you have to remember (and I think he even admitted) that he often comes up with words not for their meaning so much as for how they compliment the music, if that's what his goal is as a lyricist I would call him a successful one, and that's something many lyricists do especially in prog. It isn't really any different from what Thom York of Radiohead or Michael Stipe of REM does, they sing about gibberish too, but they make it SOUND important. And that's not a talent just any random hack possesses.
Edited by boo boo - May 04 2010 at 05:42