Seriously, the first two (Myst and Riven) were absolutely brilliant, and are still two of my favorite games of all time. The third was passable, but the fourth and fifth pretty much fell flat.

Mostly, I think it's because the characters got detailed well beyond the level they were developed for, resulting in stories and motivations that were flimsy or didn't make sense half of the time. Combined with a general decline in the quality of the puzzles, poor level design in the later ones, and the atrocious move to realtime graphics in the final one, this killed the games.

I'd like to see them rebooted with a return to the formula that made them a success in the first place: a return drop-dead gorgeous prerendered graphics, hard puzzles, intriguing levels, and a good sense of mystery. Ditch the overdone storyline and the attempts to add more interactivity to world, most of which just came off as hokey anyways.