Lost has been clever in continually threatening us and the Losties with the Others, continuing to reveal others without us ever allowing us to be sure we have found the Others. There's a see-sawing between supernatural and normal, real and imagined threats. Each set of others is at first not quite real, then as soon as we are sure they are real, they turn out to be not what we thought, or themselves in fear of the other others. It has been intelligent and fun misdirection.
The first night, even before the immediate--medical, water, food, shelter--needs of the fuselage survivors were met, the smoke monster's tree toppling activity was visible above the cliff, then the next day they are chased by the monster and see the grisly remains of its victim. The "you are not alone" theme was introduced. The Other at this point appears to be a powerful and malevolent monster.
In the second part of the same episode, a powerful monster is revealed and killed, in the form of a polar bear, but before the viewer can decide whether polar bears could have done everything we had seen, we hear the voice of Danielle Rousseau: a definite human Other. We don't know who she is, if she is alive or was ever on the island, or if the broadcast is from somewhere else.
Now that everyone is on the edge for monsters and the bizarre and malevolent, the wild boar rampaging through the fuselage scare them. Boars are predictable, natural, but still dangerous. Then Jack's dead father appears--to Jack. We don't know if Jack is coming down with the sickness or some other madness or if his father has been truly reanimated by the Island. And then finally proof that the crash survivors are not the only humans to have come to the island, when they find the human remains they call Adam and Eve.
Saïd then finds the living and present Rousseau, but before we can even breathe a sigh of relief that there are indeed others: Rousseau's people, we find that she is alone. And then she refers to others. She doesn't seem very credible. She herself says that there is a sickness that causes madness, that she has never seen the others, just heard them as whispers. The only real tangibility she gives her others is that they stole her child. But we don't know if she ever had a child. Claire claims to have been attacked by a needle in the night, but she's not entirely believed. Saïd announces "We are not alone" as he returns to the beach, but is the company just Rousseau, or are there indeed other others?
And then Hurley swiftly discovers that Ethan is not on the manifest. Before we can digest the implications of a man who seems just like them not being one of them, Charlie and Kate are captured, followed by the hatch being discovered, making the Others a real technological society working against them.
Someone burned the raft and no one knows which other that could have been. And then they find the smugglers: people who came from somewhere they no way should have been able to reach the island from. And then other others answer the radio call Boone made from the Beechcraft, and someone turned on a light in the hatch. The known other Rousseau returns and tries to steal the baby, and so does the smoke, attacking and more visible than before. Finally and suddenly the threat of the others materialized and seizes Walt.
That's a long sequence of Other threats -- and that's just the first season.