I never played L4D but wish I had. To me, B4B was also just OK. The CARD stack was a little confusing at first. Not very intuitive, but i think it got better as the weekend went on. Some them fixing it, some me getting it.
I was hoping for a Division like experience. It was more like an Arcade game where my PC felt like it was a console. I would twist and turn wildly but still get a head-shots?? mmmOk!
I enjoyed the characters and working together but it was also not designed for someone to jump in and out of. At least for the Beta I would end up playing with all Bots by round 2 or 3.
Visuals were great, but once you killed a few Zombies your team mates are indicernable from zombies because they are literally coverd in blood. Really got old being accused of Friendly fire. Which was automated so happened every single time.
Overall, it was fun as a BETA should be, but definitely didn't endear me to purchase the game. I think they should have made it go for a few more days.
I will say, if you're not playing Free Open Beta's then you are an idjiot! One of the best gaming experiences I ever had was playing Doom 2016 Open Beta!
Also, for a much more enjoyable experience with Zombies I suggest COD:CW - Outbreak Mode