The movie industry works a little differently today than it did 30+ years ago. Nowadays a movie needs to make most of it's money in the first 3 weeks, there are movies that buck that trend, but they are rare. Back in the day of Star Wars, which is still the second highest grossing domestic film when taking inflation into account, a movie could stay in theaters for months on end, and Star Wars was still playing in around 300 cinemas after 1 year. That doesnt happen today. Also, movies back then got multiple releases thanks to there being no home video market.
I'm not saying GOTG is definitely not destined for incredible receipts at the box office, but to compare it to Star Wars, Avatar or even Avengers is a long shot at best. Those movies were cultural phenomenas.
GOTG faces a tougher challenge than Avengers did due to name recognition.
GOTG faces a tougher challenge than Star Wars did due to apathy, in the days of digital cinema there just isnt a feeling of spectacle and the wow effect for special effects that there used to be.
GOTG faces a tougher challenge than Avatar did due to 3D burn out. Avatar was a case of right movie at the right time, 3D in it's current incarnation was new and exciting when Avatar was released.
I am sure GOTG will do quite well, but I think expectations should be taken down a few notches. It could be the next Avatar or Star Wars, but it probably won't be.