It would probably be a dump of a large, fully annotated and understood database system. Or there are plenty of files I have that are several hundred megabytes where the columns are x y z and are motion tracking files, which are fully annotated in perfect order as far as protocol is concerned, but if I simply handed you that file with no preparation of what any of it actual is, then you have no idea what the hell to do with it.
When we analyze that data, we know exactly what the limitations of the system that collected it are, what parts of the data are good and which parts are bad, what went wrong during that study, etc. But do to privacy issues, we cannot turn over videos of the subjects performing the tasks (this is an IRB rule, not ours) without consent of the subjects. Handing you a text file without the video is worthless. Data without context, understanding of exactly how every piece of it was collected, or understanding of its limitations is meaningless at best. At worst, it is dangerous in the hands of someone with a larger megaphone who does not understand the data (as the AGW deniers love to do).
Like I said though, I think that data should be released, but releasing data as soon as it is generated rather than after the relevant papers have been published is an undue burden on the already heavily time constrained researchers.