Ah, THAC0 was fun. Armor hit points on the other hand, I could have lived without. One thing I learned from HackMaster (which I have not exported to other games) is that everything that is not nailed down is treasure. Nails are also treasure. We left behind so many tens or thousands worth of GP in that game...
Most certainly bring your imagination. Role Playing for me is just that, playing the role of my character. Be the character, think like they would, act as they would. What we are looking at is like playing pretend, except you know the rules ahead of time. Role Playing is also more fun than Roll Playing. While some people are good to do little more than roll the dice, I desire more for my characters.
And really be ready to have a good time, and not get hung up on the dice rolls so much. You botch a roll, bummer. Find a way to work with it. Maybe your sword got stuck in the dirt. Wouldn't it be cool to do a dive roll past the opposition, pulling the sword from the ground and coming up in an attack position, ready to strike? Perhaps your bow string snapped... again. Go bug the wizard to cast mend on the string and hope it doesn't break next attack. What if your hand was lost when your magic ring blew up? You go and find a high level cleric able to regenerate it, but at what cost? The start of another great adventure I'd wager.
There are so many ways to have fun while role playing, it is mind numbing. Just remember to go in with the mind set that you are going to enjoy it, and you will.