It's _all_ about the rules. It's what makes things interesting. Without rules there can be no meaningful story. I vaguely recall an atrocious fantasy trilogy by an Australian author that didn't establish rules for the magic in her books. What happened was that _every_ conflict was resolved using the improbable deus-ex-machina device she called 'magic'. Unfortunately, I forgot both the author's name and the titles...
Contrast this with Lois MacMaster Bujold's 'The Curse of Chalion' which only works because it first establishes a set of rules and then presents a problem that seems to defy those rules. There's two sequels that continue to explore the consequences of these rules on the setting. This is how to do it!
'The Malazan Books of the Fallen' series is an edge case: While they're brilliant in many respects, the author decided to keep the reader in the dark about the rules of magic - at least in the beginning. A lot of stuff happens that you cannot make much sense of, and initially seems arbitrary. But there are rules. They just aren't obvious and are typically explained much later, sometimes even in a later novel. I'm quite sure I would enjoy the novels a lot more, if the author hadn't been so stingy with information about the rules.
Harry Potter is another interesting example: I enjoyed most of the novels except the last one because they seemed to use consistent, well-defined rules. But then in the last novel, she f**** up. The inheritable, mysterious ability to understand and speak the language of snakes is suddenly something that everyone can easily replicate simply by overhearing a few words. And for some reason it's suddenly possible to use multiple wands to increase the power of your spells. Seriously?! Why the heck doesn't everyone walk around with a golfbag of wands then - I would!
Elric, Corum, Hawkmoon, Moorcock just pulls out of his ass constantly. And it's amazing!
Now it's been a while since I read those, but I seem to recall there were quite clear rules what these characters and their artifacts could do. Corum has the Eye and the Hand, and it's established early on what they can be used for. Stormbringer is a bit more mysterious, but it's basic rules are again established early. Elric's ability to 'call on favours' from various demons is established in his backstory. Sure, you don't know exactly which demons he can call on for help, but the ability itself is established.