This game is one of the best releases this year in my personal opinion. Let me start by saying I don't even like zombie shooter games. That being said, I did play L4D2 and it will always have a place in my heart due to the feelings of excitement it provided during my younger years. Last year I picked up L4D2 again and although I had a lot of fun, I couldn't help but notice the lack of customization and dated mechanics/graphics.
Back 4 Blood hits the nail on the head for improvements. It has all the same wide eye, butt-clenching, and adrenaline-filled moments that the first has. The variety of enemies are much appreciated. Also, I don't think people are giving enough praise to the boss's enemies. Yes, they are bullet sponges but they also provide much more final act enjoyment than the L4D series provided.
Co-op and playing with friends is a must and fully brings the experience to light. The amount of laughs and screams this game will undoubtedly bring is exactly what I love about the game.
However, the biggest improvement has got to be the weapon and character system. The guns feel wonderful and are an absolute blast to use. being able to customize them also adds more enjoyment than originally expected. Finally, the characters might not be as iconic (I really miss my "pills here!") but they still find a way to bring their own charm.
The card system is brilliant too and adds much more replayability than people are giving it credit for. I've already done a glass cannon sniper playthrough as well as a tank melee. Seriously buy the game if you like fun. Otherwise buy some paint, throw it on your monitor, and watch it dry because you obviously like boredom and should be throwing away your gaming set-up at that poin.
[Mod edit: broke wall of text into paragraphs for clarity.]