Focused Studying

When it come to focusing your study as much as possible I find that focusing on the most useful aspect, and the largest hurdle to be the best way to go.

Now, I'm aware that this is a tricky question to answer, and that many people will have different answers, but I'm curious. What do you all think is a) the most useful skill to acquire in chess and b) which skill or aspect gives inexperienced (or even experienced) players the hardest time?

My vote goes to a) tactics because most chess players need to be more tactically aware and b) positional theory because it is much more difficult to study and far less fun to many.

I personally find tactics more difficult because in positional studies you note the imbalances and recall various plans associated with them whereas tactical problems can have many branches with varying depth, so a lot of brute forcing is required. 

(a) The serenity to accept complications, the courage to play good tactics and the wisdom to know when they work.

(b1) Inexperienced players? Not hanging pieces/overlooking tactics.

(b2) Experienced players? Not hanging squares (positional weaknesses)/playing accurate endgames.

Now, for training tactics you can improve just by doing hundreds of tactics problems. How would you suggest one trains/studies positional chess?

Good question. Outside of finding a decent book on positional chess (ie How to Reassess Your Chess which I think is excellent for those without a lot of experience), I'm not aware of any tools to help sharpen positional skills. I wonder if there is anything out there similar to the Tactics Trainer but focused on positional puzzles rather than tactics. Wouldn't that be a neat tool.

Isn't Reassess Your Chess geared to more intermediate level players?