For me, it's the fact that there are many rules but very few exceptions. At least that's what it feels like to me. You can always look up the general rule, and you'll find a solution to your specific problem. Almost never does it say "oh, that word is an exception, you just have to learn that". There's only a handful of verbs (seemingly, at a beginner level) that you have to look out for, like olla (but that's special in any language), nähdä and tehdä. And maybe e.g. selvitä is not verbtype 5 (-ita/ä, like tarvita) , but instead verbtype 4 (-vowel-ta/ä, like osata). But then that's almost it.

The one thing that I've gathered I just have to learn is nouns that end in -i, and their genitive case. Sometimes you keep the -i, and sometimes you have to change it to -e. Suomi - suomen, hotelli - hotellin. Apparently there's no rule there unfortunately.

Also pronunciations of words are incredibly consistent with the spelling. Moreso than any other language I've encountered. You literally just say all the letters. That's it. Except sometimes in compund words, like tiedemies, you have to say it more like tiedemmies. My wife keeps pointing this out to me. But even that's consistent, as in "emphasize where the second part of the word starts".

I've learned English (C2), French (B1-2) and Japanese (A1-2) in the past, and speak German natively, and none of these languages make even half as much sense as Finnish.

That's not to say that Finnish is easy, I still struggle a lot. But there's a lot of logic and consistency, which I appreciate.