I think the challenge that is presented to any who are drawn to the Island has mainly to do with the unique energy source there. Though some have come before the 'Computer Age', 'Rocket Age', even the 'Industrial Age', it doesn't matter (for it could be possible that the Wheel was put in place by a previous 'group' that was drawn there long ago).
So people come to the Island, find an energy/power/magic source and then seek to put it to use. Along the way human nature manifests itself- both in creativity and ingenuity, as well as avarice and treachery... ambition leading to corruption.
Nemesis maintains that it will always "end that way" where as Jacob contends that each time it happens there is progress made toward an eventual success when man will realize the various potentials, and the healing and power of the 'magic box' without turning into a complete jerk about it.
Jack's pursuit of the image of his Dad almost lead to his death, falling off the bluff that Locke rescued him from. I see this this 'cat &mouse' game as nothing but malicious, instigated by Nemesis, with the intent of Jack winding up dead.The death of the potential leader of the Losties would help to hasten the failure of the group as a whole, something Nemesis would consider inevitable
Jack found the water/caves after this was over.
Eko's pursuit of the image of his brother did
lead to his death. Smokie had been pursuing Eko all the while he trekked toward the beechcraft; in fact seemed ready to pounce on him near the creek before Locke & Co. showed up. It may be that Locke's presence stays the beast. The image of Yemi lured him away from the close proximity of Locke and into the jungle to a waiting Smokie.
Nemesis would be glad to be rid of a man who had such strength of conviction.
The image of 'Dave' tried to get Hurley to kill himself. Nemesis again. The guileless nature of Hugo Reyes would be a great threat to Nemesis' position that 'all of humanity is corrupt'.
The images of Walt got Shannon killed, creating dissension between some of the previously separated passengers. Later the image got Locke to murder Naomi causing big, important problems between the Freighties and the Losties
Appearing to Claire- To separate her from Aaron (possibly his
Appearing to Locke- To get him (and Ben) to initiate the events that lead to the utilization of the 'loophole'. Moving the Island was not necessary and was a mistake. So was bringing back the O6... initially.
Appearing to Michael- Nemesis gloating.
Entices Ben through intrigue to betray and eventually help slaughter the D.I. population, whom enjoyed a long truce with Jacob's people. We still as yet do not know what lead to the 'Purge'. Machinations of Nemesis through Ben?
Jacob seems to strive to influence people through aspects of their better nature (something Nemesis doesn't acknowledge is there). The above deceptions, images representing someone who really isn't there, I attribute to Nemesis, causing fear and confusion.
The following few I attribute to Jacob:
As I stated before this image represented Hope, even Forgiveness.I don't think it was actually Jacob per se, but perhaps a conjuration of sorts on his part, or even an actual horse (escaped from Zoocatraz, maybe) that Jacob brought to be at that place and at that time.
Locke's 'Soul of the Island':
Difficult because we have so little to go on, even the testimony of Locke himself concerning this event can be suspect. Perhaps another conjuration of Jacob's. It may have been an image shown by Smokie, displaying images (from one's mind) is one of the things that has been proven that Smokie can do. If so, might it have been an image from Locke's own mind? An abstract representation of Locke's own hopes and dreams? That might explain why Locke thought it was "beautiful".
I think the Cabin was a 'sanctified' place (made so by the circle of protection around it) where the Hostiles/Others (or at least the Leader) could always be certain that the person inside was definitely Jacob, and not some devious impostor. When the circle was broken, that certainty was compromised and the Cabin becomes useless, even dangerous.
I don't think Horace had anything to do with the cabin. He may have had a schematic of the Cabin in his pocket as a result of intelligence gathering regarding important Hostile 'areas' , something that would concern him being a staff member of the Arrow station.
More on The Numbers.
(An expansion of the idea presented earlier in this thread)