There is no future tense in Finnish. Or gendered personal pronouns. Oh and there’s about a gazillion words for snow. And god help you if you need to articulate that your spruce is returning.
As an English speaker myself it's astonishing the number and range of features that other languages have built in, so to speak. Years ago I took Latin and the idea of declension just blew my mind. And that's basic in the grand scheme.
It's also trickier with more unusual languages because there aren't resources out there to the same extent as for more widely-spoken languages. I don't think there is an 'official' Memrise course for Finnish for example.