Game play can be considered the action part of the story in the game. It's what the player can and cant do inside the virtual world. Achualy taking hold of this object (charcter) and endulging in the fantasy of being that character, being in the enviroment and so on. Gameplay is how the viewer can make choices to change the outcomes of the game. Where something like a movie is vary liniar in its story not alowing the viewer to see the whole picture, a game alows the viewer to branch off into various other paths or branches which can be played through in multipul different ways. Gameplay is the interaction of the audience to the medium and the feeling the get playing it.
Often gameplay is pushed aside in many modern games, in the therory that the graphics or story will compensate. This doesnt make an ejoyable game to play, but something pretty to look at. Games like Pac-Man or Space Invaders at the core were good games, their graphics didnt matter at all. People replayed those games over and over, nothing new came out of replaying the game, no unlockables or hidden stages came out of beating them, they only got faster and harder to complete the second time. Many games of recent times belive that replayability means you get a new item or clothing map on the main character. There is enjoyment in getting unlockable prizes for completeing something, but its not something to base a game upon. The unlocking can get pretty tiring, once one realises how often its used.
Remember what you could unlock in the original SNES Super Metroid for completing it? No? Bad, good and Best Endings for completeing it within faster time frames. Remember playing it again because you though Samus was a man, only to find out she was a blond woman? Ofcorse. Remember fighting that giant robot bird thing? Ofcorse. Did you also know they remade the original for the game boy DS? Metroid Zero misson (its pretty much the same game, few new perks and what not). The game play is mostly the same, but with improved graphics, and more shit to unlock. What does all this Unlocking have to do with game play? Nothing! So why is it the most important part of the game? Because its easier to change a character model than make something exciting to play again and again without changing anything. Why was Pac-man fun to play, the unpredictability of the insainly simple AI Ghosts that always seemed to find you, no matter how many times you played it, they'd always go somewhere else. What could you unlock in packman for completing it? You couldnt complete it, it never ends, the background changes colour when you get to a new level, thats it.
For me, when I play a game I look for exscapisum and a exploration. However many things are also important, such as the game being realistic enough to belive in and fantasy enough so I can escape into a fantasy world. I often like to go back and replay games, as this allows me to look at the games details in depth, knowing whats going to happen next so I have time to look at the details of the game. I do love a good sandbox game.