I'd say how varied it is, there's something for everyone, like I used to want to be a good standard player, then realized that it just isn't my thing, then one day I grabbed my trusty xbox controller went into CtB with controls binded to it, and discovered I'm not too bad at it and that there's potential for me to take it further. As of posting this for example I can get to the outro on Remind, by Chata, whereas on standard I can't even last five seconds.
I love the concept of skinning, I don't have the patience myself for it but love what people make.
Plus there's always the whole "holy crap I just found an awesome map for a song I love" feeling.