I replayed KQ5 a few months ago and still found it quite enjoyable. Of course, thats because I know all the answers now, the first time I played it I thought I was going to go nuts!
I hate the maze. Every time I play it, when I get there I dread it. North should always be up

I don't do well with changing directions, and it made it almost impossible for me to draw a map of the maze.
All of the games have at least 1 stinker puzzle, but the cheese to power the machine is definitely top 5 silliest things.
By the same token, all the games (except maybe 7?) have scenarios where you can miss something and then ruin the game and have to restart, but I kinda feel KQ5 is the worst offender of this. I mean, you can just wander into the forest and get really stuck. Alexander at least gets warned about the catacombs if he doesn't have the right items at the time! And if you let the rat die, whoops! It comes out of NOWHERE and if you haven't saved in a while, cause why would you, you are screwed. Don't grab the locket quick enough in the nest? Screwed. I am sure others can add to the list, but those are among the reasons that KQ5 is a little lower on the totem pole. But for me it still wins over KQ1, 2, and 7 for how fun to replay.
Mordack's castle is amazing and soooo scary, I still get terrified when I go in there. Death is lurking around every corner and to me Mordack is by far the scariest villain. The others seemed cartoony and afraid to actually kill people, I knew Mordack would kill me and my whole family without hesitation.