This list is for things that are not appropriate to post on iNat. It is a question that has come up a few times and I thought it would be handy to have the list in a single wiki post. They are not always equivalent to posting on iNat and I will go a little outside the box.
State of matter - mineral. Geology, Rocks and even fossils.
https://rockd.org
State of matter: Man-made litter. Plastic, cigarette, paper, can, bottle, etc.
https://litterati.org
State of mind Existential observations, ideas, aspirations and dreams. Inform, inspire, ignite change for the common good. (If not right here on the iNat forum)
https://commons.commondreams.org
I like this one about climate change
Astronomy
Aurorasaurus
More about volunteering to help classify than posting, but anyway
Zooniverse
GPS data like tracks or POI (point of interest)
OpenStreetMap
Landscape shots
Pics like a wetland, a shoreline, a waterfall, mountain range, clouds, etc⌠that are just natural environments and have no particular subject that is relatable to a life form. We can sometimes salvage these as an observation by finding something in there that can be IDed with some accuracy and confidence. Generally, though, pretty pictures of scenic natural landscapes are more appropriate for sites where you can just show off your photos, like Flickr (http://www.flickr.com/about).
Something I would like to see, but cannot find: anthropology. Cave paintings, middens, stone tools, ruins, historical evidence of ancient humans, etc.