The Avengers was created by Stan Lee under direction by Marvel to create a team-based book that could counter DC's Justice League of America. So, quite literally, the Avengers are a direct ripoff of the JLA.
However, if there's one thing that defines the Avengers on their own terms, it's the idea that they truly represent the idea of teamwork; of wildly different individuals putting aside their differences in order to help defend the Earth from evil.The opening titles of The Avengers: Earth's Mightiest Heroes sums it up pretty well.
Always we will fight as one
'Til the battle's won
With evil on the run
We never come undone
Assembled we are strong
Forever fight as one
That spirit of teamwork—that brothers-in-arms resolute determination to support one another and fight on—is what makes the Avengers' story one worth telling. Anyone who's ever been on a sports team, worked on a group-based school project, or even simply had a group of close friends can relate to that.