Seinfeld is the story (or rather a still life) of two sociopaths, a depressive, and an innocent; because it is a 'situation comedy,' the characters never change except temporarily, for comic effect. The purpose of the characters is to make a key point about late-20C American culture - specifically its decadence and terminal triviality. Some of the show's depth of meaning comes from its cultural specificity, its focus on a particular tribe of rich Jewish Manhattanites.
The Sopranos uneasily blended the generic demands of the mob drama with the basic form of sitcoms like Seinfeld. The shows shared DNA: The Sopranos made Big Cultural Arguments through narrow focus on one (comically exaggerated) ethnic group, too, and its main topic was the creeping pointlessness, triviality, and resulting bestial inhumanity of turn-of-the-century American culture. I don't want to overstate the links, but understanding one can illuminate the other.
Tony Soprano - who is a sociopath, a depressive, and an innocent - certainly grows over the course of the series, but one of the show's main (thematic) concerns is Tony's discomfort at being caught between oppressive, obsessively-referenced but only dimly-remembered traditions (omerta, distant Italy, blue-collar cred) and the workin' American's need to push relentlessly forward toward...something.
Drama is change, comedy is variation (stasis). The American Dream of change resonates destructively with the ritual repetition and variation that held together the culture(s) of Tony's ancestors, familial and professional. 'Whatever happened to Gary Cooper?' is one of the show's central questions; the sobering answer, 'the Soprano family happened,' was one of David Chase's oft-repeated refrains. Indeed, in service of his generational argument, Chase was often happy to let that symbolic drama supersede the personal dramas of his characters. (Hence the simplicity of characters like Paulie and Silvio when compared to analogues like, say, the simpering Farnum on Deadwood.)
The Sopranos was more like a sitcom than any of its peers among American TV drama. Its approach to its Clintonesque protagonists, Tony and Carmela - maybe the most complex pair ever depicted on TV - relied more on iterative elaboration than revolutionary change; its supporting characters were largely types, there to illuminate (or shadow) Tony's concerns; its exploration of generational transition and transformation gained much of its power from its choking claustrophobia rather than its historical sweep or daring (compare Mad Men, with its candid insight into divorce and other major midlife transformations - or even Buffy, with its yearly narrative apocalypses and constant characterological churn).
It was every bit as good as they say - a major work of art. But we should be clear about what kind of art it was. The characters who escape the sitcom of 'The Sopranos' (e.g. Melfi) find their way into the long second act of the inadequate drama that is life. One of the most amazing things about The Sopranos, to me, is that it managed to tell an episodic story (Ralph Kramden/Archie Bunker, Mob boss!) in serial form...allow its characters to recognize this weird genre trap...and in doing so illuminate the weird state of the American middle class at the turn of a weird century. Formal shenanigans of that sort rarely add up to anything. Kudos to David Chase for being, quite unexpectedly, an avant-garde comedy writer.