I don't really know.... but I guess it's the building.... I'm less interested in running about killing mobs, and more interested in building something I can feel proud of making.
I also confess to a small amount of enjoying making a home and furnishing it.... (But then I always was a bit of a fan of The Sims
In other games, you follow the story along and get the achievement of finishing a level or something, but nothing you can look back on and go "I did that" - and nothing you can look at and go "wow, that person's work is amazing".
So with Minecraft you have the sense of achievement in that you've done something you can see, something other people can see.....
Also it's always changing, there is no "way to play" - you make it what you want... build a house, dig a mine, play with redstone... do what you want. There is no right way or wrong way to play it, and vastly different people can play in vastly different ways - and play together in the one space.
I think creative/Survival has a completely different feel.... But I do like both
Building something in creative is more about being able to build what you want without the restrictions of trying to do it without dying, without needing to consider mobs and without needing to get the resources... so it's all about the build... letting your imagination go wild and seeing what you can make. Like an unlimited LEGO set.
Survival though gives the game more meaning - since planting crops has a purpose (you need to eat) gathering up animals (for food or wool) has a purpose.... mining and collecting materials all has a purpose... and you have to make sure your creations are going to keep you safe from mobs.... So while you can do big buildings too, it's a different way of doing it.
Creative is good for me when I have a project in mind and I'm building that.... but I can't play exclusively creative because I prefer having those little focuses on food and mobs, to help give the game that extra dimension.