Nas didn't change the game lyrically in terms of his style. His style was basically Kool G Rap subject matter over a Rakim level cadence essentially. The thing that Illmatic did though was to give all of these concepts and polish them to an almost perfect sheen. In other words he basically took all the best elements of NY hip-hop and used them to great effect in his own effort. In this sense I compare to a Michael Jackson or a Prince in the sense that those artists tool the best elements of artists in their lifetime and prior and melded it to create their own unique style. Nas was really the few rapper that made dudes have a greater appreciation for a rappers pen game. People think that Nas ended the so-called rappity-rap era from the earlier half of the decade but this is not necessarily true there were a lot of acts that were heading in that direction of The School of Hard Knocks, G-Rap, Wu-Tang, Redman, EPMD, Naughty was switching it up stylistically, even early 2Pac, were achieving this, but Nas did it in a short, extremely well polished album, which over time has grown to become a classic.