Possible spoilers below:
Pay attention to the number themes...show very much focuses on certain numbers. The string of numbers that Hurley got from the guy in the clinic play a huge role through the series. One example: the last two numbers were 23 and 42 which turned up again last as Jack's (and the girl he met at the airport bar) seat numbers. That's just one but there are tons of examples. There is also a theme of black and white running through the show. And for those that haven't watched from day 1, Walt (the little boy) isn't your normal kid...very important character, if not the most important in the long run.
As for the the big thing stomping around the jungle...it's basically a security system for the island that reacts to motion. If you pay attention to what happens when it's present, you'll see that it can't harm you if you don't move. This is why Locke was not killed, despite coming in direct contact with it. And last night, it was approaching but Locke said "don't move." Once everyone quit moving, it left. According to the writers, we'll get to see a VERY brief (5% or less) of the creature. There was speculation that it was a dinosaur but the writers have assured that it's not. But there is a possibility that it is part mechanical.
And yes, the raft will be having a bit of an issue...those aboard will be back but it has been hinted that one person will die. No clue who but apparently the Jin plays a role in season 2 and Walt is too important to die. Maybe Michael? Sawyer? Both are expendable as characters but Michael probably more so than Sawyer since he has become bit of a fan favorite.
Great show and in my opinion, one of the best ever because of it's uniqueness. They also did a great job with casting. Can't wait for the finale even though I know it's going to leave us hanging really hard for season 2.