Players very widely disagree with me about what's hard and what's easy. and in a way, 'I won, but it was a fight' is the best compliment a game can receive.
What I would pay much more attention to are the few points where the player can inadvertently make a career decision. Most players end up back-tracking, though some actually enjoy this.
A deliberate choice on my part was for the player to continue to find new possibilities in the early Attic rooms far into the game. I think this builds atmosphere, though it means there's no neat division of the prologue from the middle game.
The single biggest is to stop the player from getting stuck and getting bored; always think like the player as well as the designer.