For me it's a tie between the strategy of winning different types of battles and deleting 'friends' who port to me without asking. i'm not certain which one i get more joy out of. i do love the game as a whole.
My favorite part of Wizard101 is that I can make more than one wizard.
When I get bored with one wizard, or stuck in an area that I really don't feel like playing at that time, I can switch to one of my other wizards to play.
Sometimes I like my Life wizard the best. She was my first one, and she is my favorite one. I like the way that she can heal others and help them out, and she has Storm as her second school, so she has some pretty good spells too.
And if I get bored or tired of playing with one school, I can delete that wizard and start a new one with a different school combination.