The word that begins and ends with the same letter is not a plural, and it is not souls or window, though those were good guesses! (I don't know what that means. It obviously references something earlier but I can't find it) Here's another hint: One of the castaways has already seen this thing before. The others might have as well, but I know for sure that one of them, one of the main ones, has seen it before. Her last clue is that some of us might have been there this morning (I hope it is not a Starbucks).
Also, we will see what is in the hatch in the first episode but we apparently won't truly comprehend what it is until episode 3. (Don't know what that means - we won't understand what it is? Or we won't understand its magnitude or its effect?)
I've also read that Shannon sees Walt in the forest but noone believes her. And Jin, Michael & Sawyer face a new danger from the ocean.