I haven't played any L4D games, but I've played lots of Payday 2, so I'm just gonna give some of impression and preview of the game as a coop shooter instead.
Game is quite unpolished. Zero tutorial, which is bad for new players (especially idiots) who can't read UI or obj markers in game.
1) The shooting and meeleing part is kinda okayish.
You get a red indicator when you kill something, which is compensated for how zombies are kinda slow to fall of dead immediately. Like you kill a zombie, it takes a small bit of a second of it to actually fall down, so meleeing is a bit better feel for that. However, guns are definitely needed for special and mini-boss.
The mini-gun or turret in game felt horrible to use, bad sensitivity (Idk why it has different sens to usual gun) and doesn't feel impactful when killing stuffs.
Melee weapons at least feel different, the axe you swing vertically but guarantee 1 hit kill on normal zombies. While other melee weapons you need to at least aim for the head.
2) The first modern take of video games, no fricking server browsers, you have matchmaking instead. Worse, for some reasons, there was a time I got put to a server and I had 500ms+ ping. Or you get into a server where people failed mission then left, so you have to deal with stupid AI as everyone had already said. The only funny thing I've seen is joining a room late and you still 2 bots in game with the character you already picked, so total amount of "players" in a room is 6.
Also, because of matchmaking only, so you can't choose a specific mission with people doing it. You have to start your own game and pray for people to join.
Another problem when joining mid game, you don't get to choose your loadout/card (explain later below), you are stucked with whatever the bots have in their loadout. Have fun with no cards to improve you, no guns, no ammo, no medic bags, low health in horrible situation.
3) The second modern take of video games, games must have a progression goal, which is the card system of this game and also served as "replayability" or "roguelite wannabe".
It's 50-50 to me, I like it for variety of building characters, but at the same time, you are relied on other people to have a different build to maximized on harder difficulty. Which is also hoping people actually play/grind enough to have cards to build different decks.
But how do you unlock the cards? In game, in supply lines tab, the game gives you "paths" you need to work for. Each paths has specialization like better healing, more ammo for everyone, more stamina, etc...

You can click on each path to see what are the rewards, the bad thing in this progression you also get sprays which are useless imo when I only care about getting the cards. The rectangle shape rewards are cards, others are cosmetic sprays.
The red currency (supply) on top right near the party list is the currency you need to unlock each rewards in a row. You get it for each mission you completed (must be completed not fail), the longer the mission the more supply you get. I haven't managed to finish a mission higher difficulty, sad face no friends playing with randoms, so I don't know if you get more on higher diff, which I think it should be.
You get cards, then pick your preferred cards in a deck for your build.

However, in each run and each mission, you'll pick a randomized sets of card from your deck, so you probably won't be able to pick the first few mandatory cards you would like to get your build going. For example, I'd like to build a melee character focus character I need health on melee kills and may as well default quick melee using knife instead of fist, if I don't get those, then I have either praying RNG to let me loot a melee weapon in a mission or playing as Holly or Evangelo to have a default melee weapon. Holly at least to be a better choice for melee build since she regains stamina on melee kills. Stamina is used for sprinting and meleeing.
Fortunately depends where you join in an Act, for example Act 1 has 8 missions, you join later mission you'll get to pick more cards, easier to complete your build.
4) Specials enemies, there are only like 3 of them right now in game, although the game website hints more will come. The fatty gas exploding one doesn't feel dangerous enough, easy to shoot down. The small jumping one shooting web at people, it doesn't look different enough from normal zombies, you only see them easily when they are hanging on walls like Spiderman. The big one is slow but bullet spongy, better hope that you have a sniper/semi-auto rifles or guns with good recoil to shoot its weak point (head or shoulder) quickly.
5) The stuff below isn't much of impression more then preview of the game loadout system.
There is a progression currency between missions, you get them from completing each mission or looting random places. This currency allows you buying stuffs in safe house, whether it's upgrading your weapons, buying grenades/medic stuffs or team upgrades like more health/more charges for your medic bags/grenades. Only I feel like upgrading your weapon is usually the last choice, since other stuffs are better. Yeah weapons and upgrades are randomized too.

So I don't really think you actually to get the weapon you want to use immediately, but rather relying on random loot you have and work with it.
As you can see, there are different types of ammo now. Either better praying mission spawns enough ammo for your gun type or relying on your teammates to share ammo with you.
Can the game be fun? Probably yes as a coop shooter, there are team works to be required whether it's player skill or player's build/loadout. There are definitely some challenges in there on higher difficulties. But the game has many bugs, horrible AI and needs some quality of life changes (changing attachments between weapons, default melee should be knife instead of fist, etc..).
Then again, seeing TRS hasn't really got any outstanding achievements and current days video game development, it's

to see this game improved quick. About pricing and dlc stuffs, not surprising since the game is published by WB, definitely a wait after release.
Minor stuff: I don't understand the last mission of Act 1, apparently if you play as Walker (black military guy), you can load ammo in howitzer continously while other characters have to take turn to load the howitzer. Another thing I wish the game would explain properly.
Edit: I figured it out, you need to unload the around after firing it before loading the next round.