What makes it so good is the attention to detail and the sense of reality.
I think this is helped by the knowledge that all of this really happened and that these were, in fact, real people.
Robert DeNiro's character was still alive at the time of the film, so they had to change his name, but he pretty accurately portray's the real guy - Jimmy Burke - a long time theif and a gang leader. Except that Jimmy Burke was much more IRISH than DeNiro's portrayal. Years later, Donald Sutherland potrayed Burke in a TV film called "The Heist" - which was more accurate but a little softer edged than Goodfellas.
Joe Pesci's character was real too - Mob underling Tommy DeSimone, who worked for Burke. But in real life, Tommy didn't get killed because he'd killed a made guy, he got killed over money from the Lufthansa heist. He also wasn't quite the cowboy that Joe Pesci is in Goodfellas.
Henry Hill is, of course, a real person and really did go down for a drug bust in 1980.
Jimmy Burke died in prison about three or four years after GoodFellas came out, of natural causes.
Casino is good too - but it feels very much like a xerox of GoodFellas, just transplanted into Las Vegas instead of Queens. But it has the usual Scorcese "period" feel about it.
