After considering the concept of expanding bestow curse I thought of a more broad approach that might result in a very interesting low magic spell list. What if all spells that exist have no identifiable result? They do have a quantifiable rules result, but the result operates in such a way that nobody can be sure that it was actually a natural event or magic. Many existing spells already work like this. We would be dealing mostly with enchantments, charms, curses, and the like, which all operate more subtly than other spells.
I'll use true strike as an example. It has very well defined results and is thus easy to balance mechanics-wise with other spells. However, the results of the spell can easily be attributed to chance rather than magic to the third party observer. For example, Llewyrd the witch casts her enchantment on an ally by raising the dessicated claw of an eagle and whispering some strange words. This is the true strike spell. As a result, her ally strikes down an enemy. People who do not believe in magic can simply claim that it was pure coincidence and that Llewyrd's backward, superstitious ways had no impact on the outcome.
I think that, if all spells worked like this, a very effective low magic feel would result. Instead of using magic missile, summon monster, cloudkill, and fireball to destroy enemies, this new caster can give someone a heart attack, make their hands tremble so that they have difficulty holding objects (such as a weapon or shield), make an arrow easily penetrate an opponent's thick breastplate, and strike fear into their hearts so that they flee. All of these effects, in game, can be attributed completely to natural causes, although at the meta level they have very well defined and balanced effects. The magic user can claim that he caused his enemy's heart to fail (and thus his death), but people might not even believe him since it could have been a fluke.
A big advantage of this idea would be that it requires no changes to the fundamental D&D rules, just a modification of the spell list and the introduction of some new spells. Rangers and Paladins can still cast spells, but they aren't flashy spells, they are more like when crocodile dundee forces a water buffalo off of the road or when Aragorn heals Frodo with herbs. They do not necessarily seem like magic.
Healing is a tough area though, as instant healing where wounds close up and the like are not very reasonable if the effects should not be visible. I'm not sure how to handle this but I suspect that moving to a wounds/vitality system would go a long ways toward fixing this problem. Vitality can be healed up all the time using magic with no problems, but wounds would have to be much harder to heal - probably the best you could hope for would be subtly speeding up the recovery rate, but nothing instantaneous.
Many existing spells are already (or with slight modification) completely compatible with this system - aid, bless, bull's strength, cat's grace, finger of death, bestow curse, true strike, neutralize poison, cure disease, shield, mage armor, charm person, locate object, scare, fear, enervation, emotion, magic weapon, magic vestment, death ward, divination, etc.
The problem is that many of the current spells of this type are a bit on the boring side. The trick to making the system fun to play would be to create lots of fun, interesting, and bizarre curses and enchantments. Many of them could have delayed effects so that they are not useful in combat, but rather as long term effects to antagonize and/or harm someone over time, having rats that are in the area come and bite someone when they try to sleep for example or blighting a farmer's crops, whereas others could have a more immediate impact - inflicting someone with a debilitating, shooting, headache during combat or making their sword break the next time they strike with it. With some creativity and a bit of deviousness, I think that we could come up with some really fun spells.
Magic items could work in a similar fashion - nothing glowing or flaming, or communicating via telepathy, but instead just things like keen swords, cloaks of elvenkind, rings of protection, and harps of charming - all subtle and not demonstrably magical.