The point that nobody here has mentioned is that Frieren works because it completely flips the expectations of the fantasy genre. It's not a story about the journey - it's a story about what happens *after* the journey is over. This shift in perspective is what really captures people, because most fantasy anime leave you with the "and then what?" without ever answering it. Frieren instead says "okay, the adventure is done, now what do we do with our lives?" and that's a question that hits differently.
The danger though is that if you're waiting for a traditional action anime, you'll keep finding it boring. That's not the point. It's like when I watch documentaries about cats' lives - it would seem monotonous if you expect adrenaline, but it becomes fascinating if you change the way you're watching. With Frieren you have to give yourself permission to step back from the frenetic pace and notice those small details that make the characters feel real. The others are right about the writing and the composition, but the real reason everyone's talking about it is that it touches on something deeper - reflection on mortality, on what time means, on how the years change people. For an elf who lives for millennia, 50 years is nothing; for humans it's an entire lifetime. That tension, that space between two different perceptions of reality - that's what drives everything.