My main issue is how bland the plot is. The big conflict could have been easily solved if some of the characters were to use their brain or common sense for even five seconds, but for the sake of artificially raising the stakes, they have to follow along with the villain's stupid plans.
The new (good) characters are okay - they were occasionally entertaining, and I do like their backstories - though they aren't very memorable or nuanced otherwise. The villain is just annoying. Without spoiling details, the end of the story also has these two "gotchas" that kind of make you feel cheated of a more emotionally impactful ending - you'll know what I mean.
So, don't go into this movie expecting a revolutionary story. You can still expect to see some cool fight animations, and of course, Rimuru and the whole gang being badass for brief yet cathartic moments. But that's about it.
