Linearity, plain and simple. Most people hate games now-a-days that are linear, but I've realized that most of the games I have loved dearly over the years have been linear. Mega Man, Castlevania(pre SOTN), Mario, Sonic, Uncharted, Bioshock...all linear.
As someone with ADHD, I find a completely open world game to be VERY distracting, and typically, with few exceptions(Fallout 3, Saint's Row The Third), I play open world games for 10-20 hours, get nowhere due to always being distracted, and quit playing, frustrated.
I can always return to a linear game, though, and play through it. I know that I'm moving from point a to point b, but it doesn't matter to me. A focused game is one that will keep me interested.