The first of a number of entries that employ a particular mechanic that I'm a fan of, namely gravity. Delicately controlling a craft or character while working against the force of gravity and conserving fuel. This is the idea at its most basic - simply directing a landing craft onto a suitably flat section of a landscape. The smaller the section the higher the score. Coming in uneven or too fast results in a crash and failure.
Lunar Lander has been remade countless times over many platforms (I was most familiar with an Amiga public domain version until I came across the arcade original). In any incarnation it remains one of the most rewarding and nerve-wracking gaming experiences available, and proof positive of the power of simplicity in game design.