Curses are meant to be horrible, twisted magical effects that imperil a character and steer them toward removing the curse. I don't know how many cartoons I watched and books I read when I was younger where a character was cursed and they did everything in their power to release themselves from the spell. In AD&D, curses have always been lame in comparison. Usually a Curse spell bestows a -1 to hit, not good but pretty minimal in the grand scheme of things. If I'm playing a wizard, I might not even care that I am cursed.
Of course, the problem with curses in literature is that they're always unique and they always affect characters in such a way that their life cannot move forward unless they remove the curse. This is difficult to pull off with a gaming group, you would always need to tailor a curse for individual characters, or maybe even players. The other type of literary curse is the one where the cursed character is going to die from the curse. The two stories that come to mind when I hear the word curse involve dying characters: Thinner by Stephen King, and How Spoilers Bleed by Clive Barker. This is easier to pull off.
Curses should be debilitating and should hang over the player's head, making them fear the death of their character, and the easiest way to do this is restrict their character's Hit Points. Thus, a Curse reduces a character's maximum Hit Point total to half (round up) and while affected by the Curse the character cannot regain Hit Points from non-magical healing methods. A major Curse might slowly reduce the character's maximum Hit Point total as well!
This part of the rules isn't written yet. I just wanted to share this idea before I write it definitively.