Perhaps someone can answer directly. Otherwise, this might help: Gather a list of things that are and a list of things that are not d'varim shebikdusha and see whether a rule can be figured out from the examples. I'm marking this answer "community wiki" so additions to it will be easier.
- kadish (MB 55:2, from Mgila 23:2)
- k'dusha (ditto)
- bar'chu (ditto)
- Torah-reading (ditto)
- n'si'as kapayim of the kohanim (ditto)
- chazaras hashatz (N.B.: MB 55:5 says it's considered such because it has k'dusha in it.)
- Anything that we know does not require a minyan. (This is because any davar shebikdusha ipso facto requires a minyan.)
- shlosh `esrei middot (the 13 Divine attributes): The Shulchan Aruch (Orach Chaim 565:5) says that they are d'varim shebikdusha. The source is the Rashba (Responsa 1:211), but in the printed editions, he says that they are like d'varim shebikdusha. The Tur says that they are not.