Couldn't you dual-class with humans in 1st edition, or was that a disctinctly 2nd edition idea?

Anyways, the biggest "trap" you are likely to come across is that not all encounters are meant to be fought. With 3e, you get a set of encounters that you are expected to get through during an adventure. With 1e, you get a set of encounters that you are expected to get around. While some fights are simplier to just cut through, others would be easier to avoid or go around, even ones with the same "difficulty" (meaning roughly same XP rewards).