The Queen Mary's Main Hall has changed a little bit over the years, but not a terrible lot. It's still being used for its intended purpose as a shopping center - even 75 years after the maiden voyage.
The arcade contained a "smart haberdashery and clothing shop, a cigar store, a book shop and ship-to-shore telephone booths" for passenger use. In addition, Main Hall also provided access to the library, drawing room, lounge and bar. It had to have been a rather lively area when the ship sailed.
References: The "Queen Mary": A Pictorial Souvenir Of the "World's Largest Floating Palace" - QUEEN MARY (New York: Pier & Ocean Liner News Co., Inc., 1936).