In my 13 years of WoW, the war within is is Peak world of warcraft. The people who are leaving bad reviews seem like the type of people who just would never like wow regardless of which expansion it is. It is an MMO & not only that, it's a trilogy game. This means the story line is a slow drip & you will be waiting on patches to truly play the whole story. Additionally, blizzard admitted they kinda let wow die off for a bit there. However, they have no vocalized that they recognize WoW is their top cash cow with a super dedicated fan base. As such, they have been working throughout all of dragonflight to revive the game & with much success. Expect a faster content cadence, higher quality content, & more responsive devs in regard to player feedback!
The main story line & side quests are phenomenal! I highly encourage taking your time & actually reading each quest because they are awesome & really paint the picture of the zones.
Speaking of zones, they are beautiful. Granted, as many bad reviews say, 3/4 of the zones are underground which can feel a bit dreary. However, if you read the quests & immerse yourself in the story, that really shouldn't be a problem. Besides, only 2 of the 4 zones are actually dark & dreary.
As far as gameplay is concerned, blizzard implemented & crap load of quality of life changes in dragonflight & they continue that trend in the war within. My friend who came back to wow was blown away at how much more coherent & thought out everything is.
If you're a PvP'er, dont worry! Blizz is finally giving PvP more love in this xpac as well!
At the end of the day, it's an MMO & it's world of warcraft. If that's your thing, you'll love this xpac. Like I said at the start, most the people giving bad reviews seem like the type of people who just prefer RPG games over MMO's.
If you're playing TWW, take your time! Enjoy it. Don't let fomo rush you to the end...you'll miss a lot of what the xpac really has to offer!