This is the most important by far. Know the demographics, pig-out on the analytics, but above all think of your visitors ('users' is a little clinical for me) as people. People who need things, people who encounter obstacles between them and their goals.
I've never developed a great deal of empathy for people by looking at stats, but talking face to face, watching them use the product and asking a few leading questions works every time. It's astonishing how a few interactions with actual people can breathe life into the data. When you reach a decision moment you'll have all you need to make an informed decision. It might not be the right decision, but it will be based on empathy, and that is half the battle.
Think in terms of balance. Design to remove obstacles from the path your visitors take to reach their goals, and make sure that the path does in fact lead to what they really need. If you're not sure, look harder. Make these elements work and your goals for the product will follow.
Occam's Razor is a rule that contends that if you have two possible solutions to the same problem, the simpler one will always be better. Simplicity is key in website design. The days of artistically 'designing' (read ornamenting) a website are long gone. Good website design is one that is intuitive through its simplicity and self evident organisation. A good rule of thumb is any time you're thinking of adding something to the design, ask yourself if it is really needed. Question its function. If it is not absolutely necessary, leave it out. Then see if you can get rid of something else. Brevity is the soul of wit.
At one time I was the crew boss of a smallish racing boat. My job was to marshal 12 crew through a range of manoeuvres depending on the course marks and wind direction. The crew was well drilled and could handle the ropes with ease. But I noticed that efficiency suffered with the stress of the race course – anxious people tend to make mistakes or react slowly. I decided that instead of calling manoeuvres and expecting all to go smoothly, I'd take a tour of the boat before each mark and make sure sure everyone knew exactly what was about to happen and why, even if they already kinda knew. The result – stress levels dropped and efficiency soared and our boat climbed the rankings.
Asking people to interact with a website can also induce anxiety if the consequences of a click or swipe aren't clear. Let your visitors know what's about to happen and why. Anxiety will decrease and use-efficiency will soar.
For every action there is an equal and opposite reaction. That is to say, everything in the 'real world' responds when you interact with it. Digital interfaces that don't (and yes there are still way too many) can be a bit like talking to someone who isn't responding and is staring at a point in space an undetermined number of inches to the left of you. It can be disconcerting. A lack of feedback gives rise to unwanted questions like 'is this thing working?', 'how does this work?' and 'where's the nearest hedge so I can chuck this laptop in it'. In short, give visitors feedback for their actions, and make sure it's super-snappy.
In the wild west days of web design (AKA the late nineties and early noughties) designers would approach website designs from first principles, because there were no established ways of doing things, so everything was about innovation. We were even guilty of looking down our noses at solutions that seemed obvious or un-creative. Thankfully those days are behind us, and the result is a much better experience of using the web. Design patterns are tried and tested solutions to common design issues that are found again and again in all kinds of websites. They bring consistency and "getability" to the web and help people get to the resources they want. Happy site visitors means good traffic, it's that simple. If the website is boring it's not because of the pattern it's because of the content. Try not to reinvent the wheel, and only innovate if you can reliably test your innovation and prove that you really have come up with a better way.