The question "What makes those three curses unforgivable?" got me wondering if there is anything else that the wizarding world has made unforgivable. For example, are there unforgivable potions?
To restate the title, are the unforgivable curses the only things that are unforgivable?
As far as I know only the three Unforgivables are classified as such. Deadly or love-inducing potions (Voldemort's Horcrux potion; Amortentia), painful spells (Sectumsempra), or mind-altering spells (Obliviate), I think, can be used alongside the Unforgivables, but they aren't classified as such.
If you want to get metaphorical, though, here's an Unforgivable to ponder:
Broken images were racing each other through his mind: Sirius falling through the Veil; Dumbledore suspended, broken, in mid-air; a flash of green light and his mother’s voice, begging for mercy. . .
‘Parents,’ said Harry, ‘shouldn’t leave their kids unless – unless they’ve got to.’
Deathly Hallows - Page 177 - British Hardcover
"Unforgivable" in the Harry Potter world means life imprisonment in Azkaban prison. A few people have been "forgiven" of unforgivable curses, such as Igor Karkaroff, who served some time at Azkaban before naming names and being released.
Most of the potions have short-term effects or can be reversed. Felix Felicis is banned in organized competitions but probably wouldn't get you sentenced to prision. More likely you would be kicked out of the sport and probably fined.
The same is true of hexes and charms. They have counter charms and ways to cure or remove hexes.
While there is certainly behavior that will get you convicted and sentenced to Azkaban prison (whether through spells, potions, or other activities), these three curses are the only ones that basically guarantee a life sentence. That is why they are called "unforgivable."
Those three curses are unforgivable, at least in part, because there is no use for them other than to cause pain, enforce compliance, or kill.
Every other spell and potion mentioned has at least one use which is beneficial and/or benevolent.
For example, the Draught of Living Death mentioned in the comments is used to put the drinker into a deep magical sleep.
Assuming there is a readily available antidote, this would be an AMAZING tool for people in the medical profession - a potent anesthetic with limited side effects. A deep magical sleep would also help keep people from moving about and aggravating injuries, subdue prisoners safely, and give some relief to people who suffer chronic nightmares.
I'm certain there are similar uses for every other 'questionable' magic. And when such magic is used to ill purposes the law punishes them. The 'Unforgivable' curses are simply 'unforgivable', not 'the only uses of magic punished by law'.
