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.
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.
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.
This one shows us, again, that Charlie Brown is probably not Snoopy's owner yet, but that someone owns him, or where else would he have gotten that doghouse? The expectation that his yard should be mown expands Snoopy's personality further; in the middle-era, when his doghouse burned down, it would be revealed that he lost a pool table and a Van Gogh in the fire.
The dial of a rotary phone is also visible here, which things, as I mentioned before, are now receding into memory.
This is the first strip in which Schulz draws complex music notation, which he said in Peanuts: A Golden Celebration he found challenging yet satisfying to draw. Schroeder's still playing the generic baby, so the two haven't met yet.
I remember that there is a similar early Garfield strip along these lines. Garfield and Odie sing with each other. Jon comes along singing something different, represented by a different note. In the last panel, the animals shout their note at him while Jon reacts in surprise. It's one of the more entertaining early Garfield strips, I seem to remember.
It's the first use of Schroeder's trademark striped shirt, which is similar to Linus's later. It's the beginning of the character's progression out of babyhood.
Ah-ha! It is the first glimmer of Snoopy's capacity for imagination, which if memory serves began when Snoopy fantasized being different kinds of dangerous wild animals.