I only just finished the preview campaign today, so I'm a bit late to the party.
I can't add much to what has already been said, but overall it was a great experience!
Really took me back to the 90's and gave me the same feeling I had when playing those old build games for the first time.
I hope the entire game will live up to this.
The level design is fantastic, especially the first map hits the exact balance between linear/unlinear.
It does give you objectives and the need for keycards, but still offers quite a bit of freedom and of course there are secrets everywhere. I still haven't found them all.
I especially enjoyed the gameplay on the upper level of the GDF building. You can pretty much make your own route through all the offices, the cantina etc.
But since it feels like a real building rather than some kind of maze, it never gets confusing.
On the subject of level design, the shadows and lightning are extremely well done. Best I've seen in the build engine. I think it's worth pointing that out.
As for negatives, my only big issue was the performance. I was playing on a laptop so I played on classic mode. But the framerate still dropped very low at times.
I didn't have this issue with World Tour or Eduke, so it should be possible to fix this.
I also think the enemies can benefit from having a few special moves. In DN3D, most enemies had a few extra tricks up their sleeve which made them more memorable.
For example: the red guys could have a charge attack which they only use at random times and only when they're close enough.