One useful tip that's helped me over the years is that Firefox always gets the code right and IE doesn't, so if your site is displaying properly in IE and not Firefox, it's your code that's wrong. If it's displaying right in FF but not IE, it's IE that's the problem. Really useful in determining where to start looking for display errors.
Usability and Information Architecture, User Interface design, Good coding practices, Proper use of colour palettes, Semantic markup, mobile platform compatibility, cross browser compatibility...... there's a whole science behind a well built website that goes on behind the scenes and is what separates the pros from the neighbour's kid who can build you one for £50.
I stick to tried and tested layouts, not because I couldn't design something really avante garde and totally out there, but because people have very short attention spans online and familiar layouts stop them leaving in the first three seconds.