I like magic that's depicted as mysterious, but knowable. I'm not a fan of super hard magic systems where the in-universe society already has an accurate and rigourous model of how it works that explains everything, like they have it all figured out and any characters learning something new to them is still just leaning someone already known by experts 
On the other hand, while it serves a purpose in a lot of stories, I also won't get invested in the magic system itself if it's completely soft and depicted as a complete mystery that's inherently beyond mortal comprehension ... ooooo~ ... I like the characters to demonstrate some curiosity about the magic in their world; for some of them to Explain rather than Worship.
I'm also a fan of magic that doesn't feel 'too easy' to use; e.g. if the magic does not need complex external equipment/magical technology of any kind AND most people can become 'fluent' in magic before the age of 25 if they tried, then that feels 'too easy' imo
