Most girls tend to enjoy the spacial (geometric) rather than the linear (algebraic) type of make-believe, I think, but I am not one of them. I like it all as long as it rings true and keeps me entertained and guessing and isn't disgusting. I don't pay much attention to the mechanics of magic systems. It's about characters and their interactions and growth. In other words, Brandon has something for everyone, which is what makes him so universally appealing.
I defiantly prefer geometry to algebra, but I'm the only person I know who does. Contrarily I am very intrigued by the logical side on Brandon's magic systems, and since Mistborn makes the most 'sense' I would say it's my favorite. I certainly agree with your statement on books needing to keep you entertained and guessing without being disgusting, and good character development is essential to any story. I think that what makes Brandon so good for everyone is more the fact that he doesn't try to side on issues, and keeps the male/female characters fairly even(there are more men than women, but the women are pretty important most of the time). It's almost like he's trying to stay on neutral ground so that anyone can read and relate it to their view of the world.