September 17, 1951: More forthright than Bill O'Reilly

The key here is Charlie Brown's awareness of his own mental state, and the humor derives from the fact that he knows it and yet refuses to do anything about it.

September 15, 1951: Because Charlie Brown stands in the way?

It's funny, but also seems to suggest that either Charlie Brown is incredibly accident prone, or the other characters are resigned to him standing between them and the post.

September 14, 1951: The wrath of Violet

This is the first strip (or one of them, anyway, I don't remember any others) in which one of the female characters shows unequivocal anger towards Charlie Brown without cause. Later strips would focus on the feelings of the wounded CB in these cases, but here it's just used as a punchline. So even though the subject is similar to modern Peanuts, the strip doesn't yet have the depth of introspection for which people remember it.

If I keep linking every strip like this, we'll be going through it roughly in real time!

September 13, 1951: War of the Sexes, Bicycle Front

There's a Thurber-like quality to this one. Obviously, Patty is a girl who's thinking two moves ahead.

September 12, 1951: Dog vs. Girl

This one's pretty funny, but it's also worth examination due to the subtle way Schulz shows CB's relationship with Snoopy in the first panel. Of course, Snoopy's opinion of the kid declines in the coming years... as does Patty's and Violet's.

September 11, 1951: Snoopy the Lion

Snoopy's animal mimicry talents lead him towards the wild animal fancy that kindles his prodigious imagination.

September 8, 1951: Slide, Charlie Brown, Slide!

For contrast, nine years later on May 20, 1960:

September 7, 1951: Shhh!

This is not the first thing Snoopy has ever said. It is the second. (The first, if you'll remember was "Boo!")

How human is Snoopy? Later he can do just about everything the other characters can except talk, and can even do some other things they cannot, some of them apparently quite special like the "Cheshire Beagle" trick he shows off on multiple occasions. Here he's not gotten his thought balloons yet, but it's obvious he's thinking something. We're just not told what.

September 6, 1951: Another dog?!

It's a dog other than Snoopy or his brothers! This strip establishes that, for now at least, Snoopy is noticeably smaller than other dogs.

September 5, 1951: Before the fall

This strip is sort of like a middle-era strip in theme, except instead of Charlie Brown being left at the end to endure his rejection in solitude, Violet makes up with him immediately.