As it applies to new real estate specifically, you have a lot of materials, labor taxes etc to deal with. These are the most common sorts of business debts that I've been exposed to.
For existing real estate, I see value of things going up in value largely due to population increase in a certain area combined with the fed printing money (which it claims to do so constantly because the population will always be increasing?). I also see values wildly inflated due to foreign investment. A big issue in general is also due to the banking sector. I think the US government allows banks to go a little crazy with lending to everyone on anything without recourse. That being said, I personally benefit from the system allowing me to borrow money so cheaply.
Sorry if this post was a bit disorganized. I have a lot of thoughts on it and it is a great thought experiment. :)