Can you visualize a humorous sonnet? Perhaps you could have one with matter and form grotesquely inappropriate for one another, but iambic pentameter is pretty serious stuff--particularly in a strict rhyme scheme. Good blank verse (iambic pentameter without rhymes) is quite suitable for high and lofty matter. Just look at Antony's funeral speech in Shakespeare's Julius Caesar.
Alternately, can you imagine a limerick that isn't funny? A friend once announced her intention to write an epic in limericks. Won't happen. Not an epic epic. It would turn out a mirror image of Pope's The Rape of the Lock, which is humorous by treating non-epic material in an epic style. If you want epic to come off, the format has to reinforce it.
For another example, look at Instant Messenger. If anyone can hold a deep conversation over it, my friends and I can. And we try. But the depth suffers. You must chop up your thoughts so they fit into the little box:
put in just a few words
at a time
with very little
you're intoo much of a
hurry to fix.
This bodes ill for deep thoughts of western civilization.
Blogs are better, or at least can be. A blogger at least has a moment to hit backspace and perhaps even repair his entry's thought structure.
A handwritten letter, also, is different from an email. There's something satisfying about holding an actual pen and writing on actual paper--even though it is easier to repair one's grammar and spelling digitally.
A movie can say a lot of things, and a book can say a lot of things. Sometimes they even say most of the same things. But they certainly say them differently: just look at The Lord of the Rings. In the book the terror comes from that which you can't see. In the movie, Peter Jackson could hardly show you something you couldn't see, so he makes you fear that which you can see.
I daresay the Egyptians would argue that there was a big difference between just scribbling a bit of demotic on papyrus versus carving hieroglyphs into a stone wall. Why, just look (they say): when you write on papyrus you don't even form the characters properly! In your haste the birds and water merge together!
It is good to be able to speak in different ways: if nothing else, "I have become all things to all men, so that by every means I may be able to save some." But I wish that people would use these media to say things worth being said.