You guys are asking for tougher puzzles, but I can just imagine the complaints now and can't help but smile. If a puzzle is truly challenging, then not a lot of people are actually going to appreciate it. The Secret of LM puzzles were fun, but I wouldn't call them challenging. What I liked about the Secrets of LM quest was that it featured different types of puzzles.
Whatever happens in a quest, it shouldn't be easy to get it done- there should be surprises or obstacles. I also liked one part of the Mystic River quest where it required the player to work out where to drop the letter. I wished more quests in TW2 had done something similar; it shouldn't be obvious right away what the player has to do in order to accomplish an objective.
I also liked randomly finding a journal regarding the Wild Hunt in a place where I didn't expect, and at a time I wasn't looking for it. It's really boring when things in the world are highlighted in distinct ways when they are important or relevant. The game should require the player to really observe the world in order to navigate it, and do whatever.
...I am thinking quests in TW3 might already be like this, and maybe that's why the Focus feature was added. In that case, it should be possible for the player to never have to use the feature if they do not want to do that.