This game is both solid as can be and at the same time unpolished. The gameplay is great but in these days of so-called "AAA" titles people turn their nose up at the aesthetics. I think this is why metacritic gave it a low score. But in any case the game deserves some respect due to a few things that in a very noble move, it did not advertise as a feature.
One of these things is the fact that it uses netradiant and the darkplaces engine (themselves a fork of GtkRadiant and the Quake engine). Both of these are free software and licensed as such. The source code and what modifications were made to the editor and engine respectively are included on zip files with the distribution.
To my knowledge this is one of the few games on Steam that have free code and moreover ship that free code along with the install. It is very humble about this fact, and I am guessing I am one of the few people aware of this as it requires looking in the install directory.
In other words, this video game is a massive open source/free software and classic-game-nostalgia love-fest.