I agree that in this chapter we see the first contact with the unknown for the hobbits and I think it's nicely symbolised by the passage through the gate and the finality of its closing: with this act they leave behind all that is familiar and comforting to walk into and towards danger - the mists that envelope them are a clear (no pun intended !
) signal for this.
What I'm not really sure of is the evil intent of the Old Forest - apart from Old Man Willow we never actually see the trees moving or actively
doing something: I wonder whether this is just the product of the hobbits' imagination, aided by Merry's quite scary tale and combined with the strange environment they find themselves into. It might seem that the hobbits, taking with them their fears and doubts (and maybe some guilty feelings about the actions taken by their compatriots against the forest) give substance to the impression that nature is conspiring against them.
After all, when Old Man Willows exerts his "charms" the only one keeping his wits about him is Sam - the most levelheaded (and less imaginative ?) of the four.