Here's another thought. A lot of old games had really, really simplistic controls that were easy to learn for a teen or preteen. When Doom was new, you only had five controls - forward, backward, turn left, turn right, and shoot. That was it.
Call Of Duty 4, meanwhile, has leaning, aiming down the sight, the generic "action button," jumping, three stances, two types of grenades, several secondary weapons, sprinting, melee attack, and so on. Other shooters like Rainbow Six and Gears Of War take it even further and have in-depth cover systems that allow for a totally different style of movement and combat while in cover.
Think of it this way - old games were like an insanely difficult DDR or Guitar Hero level. You had to memorize the exact sequence of what to do to get through it, but you only had a few possible things you can do. Modern games, on the other hand, are like playing a guitar game and a vocal game at the same time while standing on a DDR pad, but they're designed to let you get a little sloppy and pass anyway.