The thing that made Duke Nukem 3D great was its irreverence, compared to the seriousness of its contemporaries. It wasn't meant to be "great." It was fun.
Irreverence, and richness of the environments. Mirrors! Decorations that could be destroyed! Jetpacks! It felt so much freer than prior popular FPS games. A bunch of that stuff become standard fare in FPS games soon after, but Wolf3D, if not being the first to do those things (Rise of the Triad had some of that stuff, and came out first, but was nowhere near as popular as Duke3D) definitely popularized them and was a lot of players' first exposure to some of those features.
It wasn't just goofier than the FPS games people were used to at the time—it felt like a leap forward in realism and capabilities.
