Snoopy gets his first thought balloon that contains English words.
This must have been a hard decision for Schulz. It isn't just that Snoopy is the dog character, it's that, until now, we never found out what he was actually thinking. We usually found out through his pantomime actions: laughing, angry looks, dancing, and so on. Having a character like that can be useful from a joke-writing standpoint. Now, he can just directly tell us, and so if he doesn't the joke doesn't quite work.
And yet, it is obviously a good choice for the character. If we weren't told what's happening inside Snoopy's head, then his later imagination sequences would become a lot weirder. That is to say, his thought balloons are what make those strips comprehensible, and this possible. Those flights of fancy are one of the most distinguishing and fondly remembered aspects of Peanuts. That put this strip here among the most important ones in the entire sequence.
