Possibly the most interesting aspect of The Avengers is that the member many people consider “key,” ie: Captain America, wasn’t an Avengers from the get-go: he came in at issue #4 and was granted “founder” membership after-the-fact, replacing Hulk. The latter development happened once the group became for democratic and less like a boys clubhouse that also fought off extraterrestrial threats.
Captain America coming out of the ice is one of those seminal moments in Avengers history: one of the few from these early days outside the formation of the team as a whole. That might get me some flak, but honestly: how much does the first battle with Space Phantom really effect ongoing Avengers lore? Not at all? Yeah, I thought so. Early Avengers got by on its star power, which was and is fairly impressive.
A great single-issue from the 60s run, worth checking out.