One of the major ideas of this show involves the power of faith - faith that influences the faithful for good and for ill.
At this point, the show appears to be addressing/utilizing Manichæism and more generally Gnosticism - a philosophy/theology (or theosophy, if that term can apply broadly) that essentially embraces dualism in all things, and purports to wrap most, if not all, religions into one all-encompassing suprafaith. Manichæism emphasizes that all things have the potential for good and for evil, and I believe that this includes the Island itself.
The Island isn't 'good' or 'evil,' its human perception that colors it that way. One isn't automatically 'helped' by the Island as if it's innately a force for good. Rather, more like Solaris (to use a random but apt point of comparison), the Island is an incomprehensible force - a force like the Desert God of the Israelites - and that force is both 'good' and 'evil,' in that, like the cave on Dagobah, what's on the Island depends on what you bring with you and how you choose (there's free will) to interpret the raw natural force that appears to exist within the Island.
This explains, in part, how Locke can appear both admirable and frightening, exaltant and unhinged, how his faith can curdle from worshipful abandon into a needful and ultimately petty desire for power. Healing can be helped along by the Island, but I'd theorize that in order for that to happen you need to want to heal. This may be true of physical injuries as well.
This is where the whole 'magic box' thing comes in, and it helps to explain why the Island is (a) so important and (b) so protected:
If you have faith, the Island will heal you (see: Locke, and more specifically the moments where his legs give out, and Rose). If you do not, it won't (see: Ben). This is obviously very similar to the notion of praying to/having faith in God, except that I think the larger point being made here is that 'God/the Island' isn't picking and choosing who to heal. The people themselves are, based upon what it is that they truly believe/desire. The magic box enables the manifestation of what these characters desire - it gives Locke his legs, and it heals Ben (as a child, which we're taught is when faith is most potent) and then fails to do so when Ben's become cynical about Jacob and his own purpose.
Theoretically, having a time/space jumping Island to call your own would give you unlimited power. And because it functions on its own set of assumedly scientific principles, and has no independent morality, the purpose/use of the Island is dictated by those who reside on it. Which is why Widmore, who becomes more concerned with his off-Island secrets than his supposed duty, is jettisoned. It's why Richard tells Locke that Ben has forgotten their true purpose, and it's why Richard wants to find a leader in Locke to begin with - because the Island literally needs 'good' (unattached, unselfish) people to watch over it, or it will be exploited like any other useful resource.
In a lot of ways, it seems as though the Island itself is a giant battery - a mysterious power source - and that power essentially 'becomes' good or evil or muddied through how its used by the human beings that visit/inhabit it. And I think a larger narrative point being explored by the show revolves around the fact that all of us are on that figurative Island already. Take away the mystic visions and the Polar Bears and the talk of special destiny and what we're left with are the hard choices human beings make, or don't, on the road to becoming better people - 'good' people - in the larger world.
Most religious traditions teach us to divest ourselves of the 'corrupt' world around us and to strive for a spiritually-deeper, non-materialist life of service. That's ultimately what the Island offers its visitors - the chance to attempt this without the trappings of the modern world, to connect and to evolve, to build community. But, dualistic as it is, the Island also offers each individual the chance to stop making choices, to wander Lost in the jungle of solipsism, to refuse change and embrace both literal and figurative isolation.