Default Prime author Chris Stewart looks at the relationship between difficulty and replayability through the lens of The Baconing and Dead Nation: "I’ve been playing two games this week; one of them, The Baconing, is brand new and only came out on Wednesday, the other, Dead Nation, came out on the first of December last year. This got me thinking about how my own gaming habits differ from game to game, especially with downloadable titles. I have yet to play a Deathspank game through more than once, making sure I complete every mission, find every weapon and generally do absolutely everything on the first (and only) playthrough. By contrast, Dead Nation has me coming back every week or so for another little dose of punishment and frustration. What is so different about these two games that I truly enjoy playing, which somehow results in one of them being played only once while the other one will be played time and time again?"
For me the most difficult games have me coming back. It was super hard for me to actually become good at street fighter 4 aways getting beat online but that challenge for me to become better always kept me coming back for more.
Demons Souls always had me coming back for more as well just because of the difficulty.... I love a challenge. And easier games don't give me that and those are the ones i play through only once.