I suppose because sensible explanations of it may not even exist, but untranslatable and or illogical grammar has always been the most frustrating personally -- particlarly things like creating noun phrases for reference something (no, koto, tokoro, to iu no wa), sentences that seem to have two topics ("Kore wa shuchou wa....") or an abstract topic and subject "Sore wa kono koto ga"), the use of "nan" at the end of a phrase or sentence I still have no idea about....and I'm sure other things will spring to mind after reading everyone elses headaches ;).