Looking at an old Tweet, it seems there was some consensus that it would be nice to have the NixOS iso default to using Gnome, yet for legacy reasons still used KDE.

Now, in 18.03, I came in as a new user to find this is still the case.

I'm not sure if new users are expected to somehow install in a different manner, but to make the experience more appealing, I hope it would be possible to default to Gnome instead.