(It's also possible that Marv Wolfman's style with a lot of captions and exposition made it easier to follow than today's book that are much more sparse. Or, again, maybe I just already liked super heroes!)

It was great.

Going forward, I acquired the odd Spider-Man comic here and there, each adding tidbits of knowledge about the character, which I devoured.  

Eventually, Marvel UK introduced a B&W Spider-Man pocket book, which reprinted Amazing Fantasy 15 (and the subsequent ASM issues) filling in the blanks for me.

Far from being put off by the character's extensive back story - I could not get enough!

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I agree. Some characters (Spider-Man, DD, Moon Knight, Ghost Rider) just did not belong on a team book in the first place.

Without that core group it becomes what the Defenders, Guardians of the Galaxy and Champions have become - groups thrown together using another group's name.

Making Spider-Man, Brother Voodoo, Blade, Doctor Strange, Luke Cage, Moon Knight, the X-Men, the Fantastic Four or Shang-Chi members takes them away from their own niche in the Marvel Universe and homogenizes all the heroes together.  There used to be some uniqueness with street level characters, mystical characters, the mutant community, etc. but no more!

I do think a lot of people tend to say "new reader" when they really mean "lapsed reader."    

But I also think a lot of comics these days are not really "new reader friendly" in that they don't seem to cover the basics - characters aren't named, motivations are vague, etc.  

Agreed, but if anything, I think having them join the Avengers impacts the uniqueness of those characters individually, rather than hurting the Avengers franchise.  Having Spider-Man as an Avenger doesn't make the team or the book objectively worse.

As far as the Avengers being an exclusive club of Marvel's greatest or most powerful heroes is concerned, that hasn't been true since Avengers #16 in 1965.  

As Robert mentioned, there's always been a mix of Earth's Mightiest Heroes, along with others who had much less impressive powers or resumes when they first joined the team.  Many members were new characters or c-listers who rose to greatness because of their time in the Avengers- not as a pre-requisite.  

For your consideration, here are the "classic" Avengers who joined after Cap during the first 200 issues:

If a long-time writer/artist team on SPIDER-MAN had a brilliant idea for an epic that required the culling of 10 or 20 of the worst Spidey villains from his 60-year history, sure go ahead!

That last JAMES BOND movie basically killed off everybody--just because it was the last Daniel Craig Bond.  Halfway through the movie, I though "Oh jeez--it's one of THESE!"

CRISIS ON INFINITE EARTHS was mentioned above, the most famous and important "culling" in comics history!  They killed Barry Allen, Supergirl, the Crime Syndicate, Kamandi (erased from existence in favor of Tommy Tomorrow--who was never again used in any impactful way), and actually the Huntress.  And then--they all came back!  In confusing ways.  That's the other flaw with a culling--they lead to rampant resurrections.

I agree with JB about bad characters--just stop using them!  And then who knows?  Some good writer and artist might just find some way to use that "loser" and make him a star!  (Englehart and Rogers and Hugo Strange and Deadshot anyone?)

Guilty. I don't care so much about Shang-Chi, Ms. Marvel, or even, really, the Falcon and/or The Winter Soldier as featured characters. 

I was much more excited to see Power Man and Iron Fist team-up in the Netflix Defenders series, by contrast. And I quite liked the She-Hulk show.

I can understand indifference toward some characters, as there are so many - even in live action - now. What I don't get is the hatred that's been spewed toward a few.