Ben, Raiders is my all time favourite movie and there are so many great parts of that film which look fantastic on a big screen, so here goes...
I did notice some things that I don't normally see on the VHS tapes. When Indy realises that Marion is alive, I could have sworn I could see tears on Indy's face, which I thought was great as it shows how much he cares for her. Another thing I noticed was when Indy falls into the Well of Souls, the glass between Harrison and the snake is not visible, which makes you wonder why you can see it on VHS.
At the end, just after the Ark lid comes down and Marion and Indy hug, keep any eye out for the Ark as it almost seems to glow.
Generally, the greatest parts are the whole opening sequence, and especially when Indy runs from the boulder, the fight scene in Marion's bar, generally all the fight scenes, not to mention the whole Indy shooting Arab assassin scene! The scene where Indy goes after the Ark truck is fantastic to see on the big screen and along with John Williams' score, it's magnificent! Oh, and not forgetting the scene where Katanga and his men are looking for Indy, and our hero appears on a submarine, the crowd cheers, JW's score is thundering away *sigh*.... and the end sequence is just superb!
Hope you have a FANTASTIC time!