JUMPING IN HERE TO SAY: As someone who has played Smash competitively, I think it's a real disservice to say that it's merely the items and stages that make Smash uniquely different. The actual mechanics of Smash are unique in themselves - it's not a game based on health reduction, it's a game based on ring outs. It's more of an endurance game than other fighting games. Since your pool of moves is limited, you have to be creative to hit your opponents and get them off guard to throw them off the stage. (compared to the long move lists and myriad techniques to win of Street Fighter IV per character for example, Smash is kinda refreshing as fighting game cause it brings you back to things like spacing and thinking tactically without the complexity of multi level movelists)

That being said, I do hate playing Smash competitively because it is rather joyless (fox final destination no items is circlejerking at its finest, but all characters final destination no items is a lot more fun) but this mode is a long time coming. Nintendo and Sakurai have pointedly refused to acknowledge any sort of competitive scene for Smash until relatively recently and to have these different mode shows that Nintendo is listening to its fanbase, all aspects of it.