+1 for .idml support (import AND export), especially if a spec has been published.
I'm sure this has been discussed ad nauseum already, but very few users of this suite will have the luxury of operating in a vaccuum where they do not have to share files with other agencies, service bureaus, and clients. In order to gain traction as an alternative in an Adobe-dominated world, it has to be easy for new users to pick right up where they left off with existing files, and collaborate with other people using Adobe software. It can't be a hassle or a special request for different formats every time files need to be exchanged. It's bad enough if you're on an older version of Creative Suite and have to ask someone to save back from Creative Cloud to an older format just so you can open their files.