When I started with adventure games, I was playing stuff like Monkey Island 1 and some modern ones(aka illogical inventory puzzles. God help me.) When I played Maniac Mansion, I already knew the interface, but the one thing that made it stand aside for me was how you could switch between characters at will, and how this game required a sort of strategy to get through, not just solve this puzzle, then the next, etc., etc. I've never beaten it, though it's still one of my favourites. (I'm probably the only person my age who has even HEARD of an adventure game.)