As I am a huge fan of Lovecraft based games, I'm very interested in MoM. However, I'm curious about the role of the Keeper: will he learn so many "secrets" that it will prevent him from enjoying the game as a player (due to knowing too much about the scenarios), or are there enough random elements in the game to keep it unpredictable even for someone who knows the scenario?

You can just do them one at a time and then switch off with someone else.  Not reading ahead is going to be all up to you.

Yog-Hurtoth said:

It looks like the Keeper gets to make many choices during setup, so instead of random elements (although there may be random elements, the keeper setup was not released), I imagine you can play as an investigator in a story you have run since you will be trying to figure out how the Keeper setup the story.

Yes, there are some random elements to the Keeper/board set up, from what I could tell from the game I played Sunday.

 When the Keeper sets up the clues, he makes choices from a list that determine where to place the corresponding clue cards. Will it ruin the enjoyment later? I doubt it. There are so many permutations that playing the exact same scenario is unlikely. Even if it did occur, with a different keeper playing they will spawn monsters and achieve their goals in a different fashion, with you still trying to find out which one is yours.