I think the difficulty of Riven is definitely cranked up a couple of notches too high. Myst seems to hit the sweet spot perfectly.

I felt like there were far too many moments in Riven where I didn't look in just the right place (the space at the bottom of a door, for example) and so I couple not progress. I played the original 5 CD version where you had to swap CDs as you moved around. Further into the game it finally requests "Insert disc 5". Somewhat fittingly for Riven I couldn't find the fifth disc because it was hidden behind other discs in the double CD jewel case....