I get how you'd be underwhelmed by it today, if you're young. Personally, I think it's the greatest dramatic series ever made, still to this day.
And yes, that has pretty much everything to do with it's contextual impact on television in general. It single-handedly changed the game. It's the reason we got Lost, and Dexter, and The Shield, and Breaking Bad, and Sons of Anarchy, and The Walking Dead, and Game of Thrones... And now, 20 years later, about a thousand other series that have technically surpassed it in terms of scope and in polish, and which star A-List actors.
But NOTHING will ever surpass its ambition, or what it accomplished.
No, Oz didn't change the game. But you're right in that I probably shouldn't have used the term "single-handedly." 'Oz' was HBO's first attempt to change the game. It was great, but didn't capture the attention of the nation like 'The Sopranos' did. Hell, even 'The Wire', which is ARGUABLY a better series than 'The Sopranos' (I don't think so, at all, but many would debate me on that), didn't escalate the esteem of television.
It was the national obsession in the early 2000's with the cultural phenomenon that was 'The Sopranos', which gave rise to the big-budget, highly serialized, highly cinematic television series that more and more attracted A-List acting talent.