I would say unrealistic sellers and even more unrealistic buyers.
Buyers come to us with pre-qual letters that aren't worth the paper they are written on, and we have to break the bad news to them when they don't actually qualify. If they DO qualify, they won't listen. They heard what they wanted to hear on HGTV or found some 'guru' on the internet that knows better than the professional real estate aent they hired to help them. They think they are going to get granite counter tops and travertine marble and they are going to pay 20% less than the home is worth to boot.
Then we have to deal with sellers who think their home is worth more because they OWE more. And often we have the same problem as we do with buyers, they do not see the value of having professional representation. They think they know our job better than we do and that all we can offer them is access to the MLS which they can buy through some discount broker for a song and a dance and when that doesn't work, they blame ALL realtors, even the full service type.
My solution is to be more discriminating about who I choose to work with. Not every buyer or seller who comes along gets my time and energy. Still, it is frustrating to say the least! And working with the banks on short sales, whether you are a buyer or a seller, is a NIGHTMARE!