To start, a recap on _most_ subpanels: For most subfeeds, you run an equipment grounding conductor that is separate from the grounded conductor (separate ground and neutral), and then at the subpanel you must have a _separate_ ground and neutral bus.
The reason for this is to prevent 'objectionable currents' from flowing on the equipment ground system. Essentially you want to prevent the equipment ground system from operating as a parallel conductor to the neutral wiring.
The one exception to this rule is for a separate building, with a number of other restrictions. Basically, if the only 'parallel conductor' would be the earth itself, then you are permitted to have this parallel ground path. This means that you can have no _bonded metallic path_ between the two buildings. If you have a metal water line between the buildings (which would be bonded to the ground systems), then this would form a metallic parallel path, and you would have to separate the ground and neutral bus. If you have a communications line with a grounded shield, or a fence attached to building steel at both ends, same deal.
Finally, there is an additional requirement: if there is ground fault protection on this subfeed (either on this specific feeder, or on the entire service), you must run a separate equipment ground to the subpanel. The reason is that depending upon the design of the ground fault protection, rebonding ground and neutral might either result in false tripping or desensitization.
But if all of the requirements for not running an equipment ground conductor to a detached structure are met, then you can either run an EGC and use a 'standard' subpanel, or you can not run the EGC and have a 'service like' panel. This is now a _design choice_.
_Personally_, for _short_ runs I would run the EGC and have separate ground and neutral busses; this allows for more flexibility in the future, and for _long_ runs I would just run the neutral and rebond at the detached structure, because IMHO this would probably be better for lightning protection. But those are just hunches, and the dividing line between 'short' and 'long' is quite fuzzy