The engine plays a huge, huge role in how an FPS 'feels'. If they used another engine you would immediately notice a total difference in the game play.
I am in the beta and it feels just like CS:S. It's a fun game, but I'm a little torn on it. Feels just like CS:S with a texture mod and smaller player counts. Kind of like, what is the point ? Is the feeling I get playing it.
The Source engine may be dated, but it's one of the best FPS engines for competitive play. The hit detection is better than most every other shooter out there. Looking squarely at BF3 when I say that, terrible hit detection and lag compensation in Frostbite 2.