It's a bit of an old-school style, which makes sense considering it's a 29 year old project, but I wouldn't even say it sounds particularly dated.
The production is simply put: ICONIC! Very melodic, heavy on keys, and tons of vocal samples as well. Very chill. It's just very pleasant and easy-to-stomach, which actually really contrasts with more recent hip-hop, which can get pretty industrial and loud with it's production. Not Illmatic. I think it's what they call boom bap, but I'm not particularly knowledgeable about genres/styles.
The rapping is also immaculate, of course, it's Nasty Nas after all. He's very technically proficient and pulls out a few different flows, though I must say he goes for a very 1994 style every time. The lyrics don't fall behind either. Plenty of fun and sharp similes every verse; notable exception being the one about being bisexual.
The message is great too. A lot of love for New York and talking about living and growing up in New York, a lot of bragging and talking about his success. He talks of his struggles, but also finds time to brag about his abilities and success.
One thing I will say is that the chorus are usually not very good. They're not horrible or anything, just not as good as everything else.