Any one of these tips could be the most important if they're the "missing link". Most often when you're dieting it's a series of missed opportunities that ultimately ends in failure or slow results. The goal would be to focus in on all of these tips and in doing so eliminate the variables that cause these failures. Being perfect isn't a necessity, however, improving your lacking areas just might be.
Tip #7 is staying hydrated. I know you've heard this one a million times, but this time it's going to stick...right?....RIGHT?! Staying hydrated is so important because it has a cascade affect across all cells. It has been proven that being dehydrated will lead to increased hunger. Your body is incredibly smart (even if you aren't....kidding, calm down) and it knows that most foods contains a large percentage of water. It will increase thirst and also hunger at the same time. This will cause hunger that may lead to snacking unnecessarily, which could sabotage your diet. If this wasn't difficult enough then your poor hydration will also slow lipolysis (fat breakdown). If that wasn't already a real kick in the rear, your body isn't finished sabotaging you yet. Dehydration will also affect your cell mass which can decrease insulin sensitivity and in doing so slowing glucose/fat entering the cell to be used by the mitochondria (energy factory).
Here's the next level of suck. If you're exercising then your blood volume can be reduced and with this comes lowered blood pressure. Increased blood viscosity will lead to slower oxygen/nutrients delivery to the peripheral skeletal muscles. Organs will always be a priority (especially the brain and heart) and this will result in reduced exercise potential.
The amount of water to drink is still up in the air. I personally would shoot towards a gallon of water a day for the majority and for athletes this would be a minimum standard. You will be getting a decent amount of water from your food, but I would error on the side of caution and aim high! Drinking that much water is not dangerous so don't even say it (just don't drink it all at once)!
Prioritize your protein, get enough calories (nutrients), don't mix fuel types (carbs/fats), add exercise slowly, and drink your damn water!