Do you have any perticular ideals concerning the depth an author gives to a system that plays a key point in a story? An example being, magic; does it need a certian amount of depth, or can it be an answer to something that doesn’t concern the plot heavily? Ect.

That’s a really big question, and one I think I’ll save for a metawriting entry, but here’s a light discussion of the matter. However, the short answer is that a system should be robust enough that no action becomes deus ex machina. However, the system itself needs to avoid becoming too complex, because then the system overwhelms the narrative. 

Think of it like a video game. Video games have rules–programming rather–which determine how characters within them operate. If those rules are too shallow, we as players are often thrown out of the realism, say, when we come across doors that don’t open, invisible walls, weapons we can’t operate for inane reasons. These poorly defined rules–i.e. they apply where they reasonably shouldn’t, or vice versa–are poorly defined systems.

On the other hand, a system which becomes too complex overwhelms the narrative itself, or becomes the reason for the games existence. In some cases, such as simulation games, this might be appropriate. However, when there are too many rules to abide by, or rules are created arbitrarily to simply flummox the player, these overly complex systems can be just as game-breaking as poorly defined systems are. 

There is, as in all things, a sweet spot. Keeping with the games metaphor, one of the few I’ve seen which hits the sweet spot perfectly is this little gem called Spelunky. It’s a rougelike dungeon crawler, but the rules are simple, and apply universally. Does an arrow trap trigger when you fall it its line if sight? Then it will trigger when anything falls in its line of sight. It is a game that allows for unforeseen consequences of actions that ought to be predictable, but those consequences always result from a system which is transparent and understandable.