It sounds obvious that a plot should be interesting, right? However, Metal Gear Solid 2 sold about a million less units in America than its predecessor, despite greatly enhanced visuals. Many people felt this was in no small part due to problems with the plot. Quite a few American players regarded it as silly.
There's no shortage of boring, cliché plots out there, and plots with no emotional engagement. I and The Freeman Group have been called upon to handle these problems on more than one occasion.
The truth is that coming up with a great plot, whether it be in a film or in a game, usually takes a tremendous amount of time, thought, creativity, and work.
Furthermore, the plot should unfold in interesting ways, with plot twists, unexpected revelations, causing the player to reassess what's going on or adapt to changing circumstances. You want the player to always be dying to see what happens next.