As a developer who works with designers alot (cukerdesign.com), the things I have found most often in contention are 1. white space, 2. when fonts Need to be a certain way, and 3. colors matching (often with photoshop, comps and such, colors get a bit altered due to colorspaces).

That being said, I think it is much easier for a coder to pick up on what is important for designers than vice versa.