I've been rereading the Avengers from the beginning - for the first time in, oh, 30 years (!?) - and it amazes me how quickly and thoroughly they changed the membership, yet they still retained a loyal readership. Hulk gone, Cap in, then everyone EXCEPT Cap gone and Hawkeye/Quicksilver/Scarlet Witch in, then Giant Man/Goliath back, Cap gone again... What a revolving door.
The other thing I've noticed (which I didn't really pay attention to when I was much younger) is how in Stan Lee's scripts, every one of his male characters talks with the same groovy vernacular, whether it's Captain America or the Hulk. He wasn't the greatest at dialog, was he?
I'm about to start rereading the early Avengers myself. I agree that it's surprising that the book stayed popular with a fast changing line up and style. If you go through the series in 20 issue leaps, the "feel" of the book has changed significantly with the line-up. I think that kept it feeling fresh and exciting, compared to such "static" teams as the FF & X Men.