Solitaire is a simple game included with Microsoft Windows. Many other operating systems include similar games. Solitaire is what could be called a "time-waster" game. It is designed to relieve boredom (or avoid work) for short periods of time. It is also the first computer game many novice users see.
Since it is used this way, Solitaire's UI must be extremely easy and intuitive to use. The user will not want to put much effort into learning the interface for a simple card game.
Solitaire's UI is based around, and makes excellent use of, direct manipulation of UI elements. To play the game, a user must move cards from one stack to another. To move a card, the user simply clicks on a card, and drags it. If it is dropped reasonably close to a stack, it will be placed on that stack.
This kind of direct manipulation is good because there are few concepts for the user to think about, or for novice users, to know, before they can play this game. It is simple, and successful, and there are very few ways to improve upon it.
One improvement could be how the game handles illegal moves. Currently, if you try to place a card upon a stack it cannot go on, the card is simply moved back to its original stack. I would not suggest changing this behavior, but a simple improvement would be to use the status area at the bottom of the window to inform the user of the relevant rules of the game when they make an illegal move.