Do wonder if Vermintide 2 would be regarded as something of a modern masterwork if not for a couple asinine design decisions which gatekeep new players from having fun. Picture yourself designing a game where it takes double-digit hours before the game becomes remotely challenging or interesting.

Many reviews have noted how brainless Vermintide 2 is. There's veracity to such claim, because it's true on the lower difficulties. It speaks to how needlessly prolonged and detrimental the progression system is, causing people to abandon the game before nigh everything which makes the game fun to play can be experienced. Things like monsters, dangerous elite enemies, hordes the size where they can pose a real threat, ect... are locked behind a difficulty level not immediately accessible. It's understandable why a game wouldn't swarm a new player with everything all at once, but, apparent by the reception, the training wheels stay on far too long.
Which is a great shame, because the game is amazing. Playing through the (rather large pool of) maps with different characters will make for a completely different playstyle and role in the team. There's a real learning curve in mastering each class' specific abilities. The way the classes are set-up demands teamwork. On the highest difficulties, failure is always a couple mistakes away. The characters are charming and well-written, with several hours of recorded banter between them. The narrative takes the world of Warhammer sincerely. The game is littered with small attentions to detail that show the developers adore their own game. Audio integration into useful gameplay information is immaculate - among the best in any game I've played. The pacing of the levels is superb; the escape sequences being particularly memorable. The developers have created a great deal of new levels years after launch free of charge, as well as an entirely new rouge-lite game mode. One can hop on and experiment with new ways to approach the same level, even after hundreds of hours.

Vermintide II leaves a lukewarm impression, but is immensely gratifying beyond the surface.

Without a doubt the best multiplayer game I've ever played. Initially, while playing on the lower difficulties, the game feels like a braindead horde killing game, but as you keep playing and increase the difficulty, it becomes apparent how important communication is, how a good strategy is crucial for survival, how complex and deep the combat actually is.

Completing 80 weaves was a brutal experience (done in Season 3), but by finishing those I went from struggling to play with a group on Legend / Cata, to being able to clear missions solo on that difficulty. It is rewarding, it is visceral, it is fun.

The game is most fun when played with a fixed party using voice chat, but can also be enjoyable when joining quick games.

I wanted to play this game for a couple of hours just to see how it was and now I'm over 80 hours on record. I really needed to relax so it's probably for the best, but I digress.
Vermintide 2 is a straight upgrade for Vermintide 1, most of the core mechanics make a return so the biggest difference is the new content. By this I mean new classes, maps, challenges, etc.
As a result for anyone who didn't like the first one the sequel is unlikely to win them over but for those who do there's pretty much no reason to play the first game over this one. As I said, it's a straight upgrade.
However, for people who don't enjoy games like Left4Dead Vermintide might just win them over thanks to the focus on melee combat. Lower difficulties might lead you to believe that spamming left click is all you need in order to win but you quickly learn that levels and talents are far less important than the moveset and properties of your current weapon. It's not too complicated when you break it down but being able to land headshots consistently and staggering certain enemies makes a world of difference. In short, the combat is largely skill based, a lot of fun and had enough depth to keep me interested even after the 10.000th Skaven slave kill.
Another aspect which I found endearing were the Ubersreik Five, also known as the playable cast. They have a lot of personality and the casual interactions between them are just overflowing with character. It really makes you give a damn about what you are doing in each mission, especially if you are anything like me and prefer playing with bots most of the time.
On that note, the game can't be played offline which is a real shame since that is possible in the console version. I would really like if it was for conservation purposes because I really like this game.
About the story, you don't really need to play the first game in order to understand what's going on. If you want to get into Warhammer Fantasy don't blame me when you are reading wiki articles at 4 AM. It's a lot of fun though and enhances the game immensely because there's a significant degree of attention to detail when it comes to made-up lore.
Finally, the game has some DLC but it's downright generous by current standards (and compared to Darkide). Most of it is cosmetic and I think the characters already look pretty good by default. Also, what classes and weapons are meta change with the time so it's not like any of it is mandatory to stay relevant. Right now as I'm writing this the strongest class is Witch Hunter Captain which you unlock after beating the tutorial.
Anyway, this game is great and I hope it doesn't die when it stops being profitable to keep the servers up.