This is one of my favorite Peanuts strips, a topical joke on the fact that the big leagues had the mounds lowered after the 1968 season (when, as Charlie Brown accurately states, pitchers dominated the game; so much so that the American League batting champion hit .301).
It's been great seeing the Peanuts website running the 1969 strips; they go backwards -- last year they ran 1970 strips, and 1971 strips the year before that -- and now we're finally getting into Schulz's golden era, the '60s. I think you could make a case that 1969 was the last truly great year for Peanuts. The 1970 strips had some good moments, of course, but they were incredibly Snoopy-dominated; almost every strip and every story was about Snoopy. There's more good Charlie Brown and Schroeder material in the first month or so of 1969 (they just ran the story where the Kite-Eating Tree eats Schroeder's piano) than there was in all of 1970, or so it seems anyway.
There was still a lot of good stuff left in the strip in the '70s, and some of the best extended stories, like the "Mr. Sack" series, come from that decade. But it was around 1970, when Woodstock got a name (he'd previously just been referred to as "that stupid bird") and both daily and Sunday strips started to get Snoopy-centric, that Snoopy clearly displaced Charlie Brown as the central character of the strip, and the strip suffered for it. Even Schulz mentioned around 1972 that the main complaint he was getting from older readers was that there was too much Snoopy and Woodstock -- and to his credit, he responded by creating more long stories focusing on non-Snoopy characters. (The long stories, however, became a separate problem when they started to focus on Peppermint Patty to the exclusion of everyone else.)
Speaking of the Schroeder/tree story, this strip is just plain brilliant. That last panel cracks me up every time I see it, and it's also a model of how to balance separate actions within the same panel, as well as how to get laughs from actions that are kept outside the panel (that "chomp chomp chomp" from the tree). I also love the fact that the kite-eating tree, which started out as Charlie Brown's bizarre explanation for what happens to kites that get caught in trees, here becomes a literal reality that all the other characters accept. It's like for once, all the characters are living in Charlie Brown's own bleak little world, where everything, even nature, is out to get you.