Quoting these for clarity:
Flopsy said:
rape, scat, furry -rating:s, loli, bestiality, gaping, pregnant -rating:s, large_insertion
I'd also consider including shota, yaoi, futanari/newhalf, abuse/bullying/sexual_harassment, imminent_rape/after_rape, corruption/brainwashing/hypnosis, vore, amputee, peeing/pee/golden_shower, gangbang, sex_machine/milking_machine, inflation, nipple_penetration, virgin, menstruation, prostitution, bukkake, public_use/cumdump/immobilization/human_furniture and fat_man/ugly_man. These suggestions are all based on estimated level of user dislike, no condemnation or disrespect is implied.
Some entries may be qualified with -rating:s or rating:e. In fact, I think it might be a good idea to add rating:e by itself as the very first entry. People who are looking for raunchy stuff will probably be stubborn enough to find and remove it.
To make the default blacklist easier to understand, it would be helpful to add a link below the blacklist field in the settings page. Clicking that link would open a help page (in a new tab, if possible) that explained what the blacklist was about and how to edit it. The help page could also contain an auto-generated list of links to the wiki page for each tag on the default blacklist.
Like someone said, there should also be some kind of persistent but dismissable notice (e.g. in the announcement bar at the top of the page, or where that "Upgrade for only $20!" banner appears) for all new or anonymous users, informing them that a default blacklist is active and containing a link to the settings page where they can edit it.
Edit: immediate_rape => imminent_rape. Thanks hemogoblin!