Written so the greatest comic strip ever drawn will get a good writeup on Everything.
Before Peanuts... what was there? There were a few good strips, especially Thimble Theatre (the birthplace of Popeye) and Krazy Kat, but beyond that, really, what was there? The answer is, not much. Many deride Peanuts because they have not been exposed to what comic strips were like before. There were a few good strips, but for the most part everything good there is to find in the modern comic strip is there because of Charles Monroe Schulz.
Peanuts pioneered the idea that you could actually have intelligent content in a comic strip. Some sneer of a perceived tendency towards simple happy moralism, exemplified by the phrase "Happiness is a warm puppy." These people seem to have missed the darker undertones of the Peanuts environment, and have never read the series in which Snoopy discovered the birds he allowed to hold meetings in his doghouse were in fact evil. (The discovery: Snoopy eavesdrops in the third panel of one strip, and is grief-stricken in the fourth: "That's the most frightening thing I've ever heard!")
In brief: Peanuts is about a group of kids and a few animals and their incongruously mature outlook on the world. The main character, everyman Charlie Brown, is a constant failure, often mocked by his friends, but never gives up. In contrast, his dog, Snoopy, has a vibrant personality and inhabits a fantastic, surreal reality of his own. Charlie Brown's friends Linus and Lucy are an antagonistic brother-and-sister pair, Linus the quiet introspective one who can quote from the Old Testament and cannot survive without his blanket, and Lucy the ill-tempered, overbearing sister who makes his life hell. Other cast members include Charlie Brown's sister Sally, young Beethoven enthusiast Schroder, best-friends Peppermint Patty and Marcie and Snoopy's boon bird companion Woodstock.
Humor is a subjective thing, and yes, in its later years more and better laughs could usually be had by reading Calvin and Hobbes. But Bill Watterson himself admits the debt he owes to Peanuts, and C&H did not have to last for almost fifty years' worth of content. Few realize the immense creative drain a daily comic strip can be, and Peanuts was still funny and intelligent right up until the final days. How many other strips can last half a century and still make that claim?