What is excluded when importing IDF into Open Studio?

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Actually if you just want to swap out the HVAC system, you can import the IDF file into OpenStudio user interface without going through SketchUp. You would find that the HVAC system in the original model is going to disappear and you can redefine them in openStudio. As far as comparing the OpenStudio results and the original reference building model. In addition to the missing holidays, the object "Site:GroundTemperature:BuildingSurface" in the original reference building is going be missing, which could contribute to some differences. For the 3x higher energy use from the OpenStudio, I suspect that it may result from the air loop being sized as 100% outdoor air. See this thread. If that is not the reason, you can always compare the original reference building model IDF file and the OpenStudio generated IDF file directly, which may take some time.

Here is a link to imported objects. If any schedules or constructions failed to import you would have a failed simulation because of missing object.

After importing an IDF into the SketchUp plugin you can save an IDF of the un-translated objects. This can be accessed by the "Export Untranslated EnergyPlus Idf" item under the "Extensions/OpenStudio/Exprot" menu as shown below.

You will see difference of a few percent in OpenStudio because special days/holidays are not included. I would look through your results and see if there are 1-2 zones responsible for the 3xhigher cooling or if it spread out across all zones. Something that big should be pretty easy to track down.

For reference, we are working measure to make the DOE Prototype buildings as described on this thread.