Consistency is king. By that I mean a website that has glaring changes from page to page, changes such as major color shifts, body text in a different font, of virtually any of thousands of things that make the "jump" between pages feel un-natural, is by definition a bad website. If your going to have a bad design then at least make it consistent throughout it will improve the overall effect of the site by a large percentage, and help to offset the weak design itself. Always be consistent as possible on fonts, colors, and layout, if the color must change from page to page then so be it but make it feel like part of the design and not a mistake. Font and layout shouldn't change for any reason they are too jarring when they change form page to page, color can be worked to feel natural, font and layout are much less adaptable.
Standard Navigation is also a major player. When the navigation of a site is confusing, i.e. moves around and is lacking of pages etc, the site generally speaking is regarded as useless, annoying, or too much trouble all of which amount to a user abandoning your site. Navigation should remain the same on every page through out a site, being in the same area and consisting of the same buttons, it also should be able to direct a user to every page within the site and back again without the user having to fear getting "lost" in the site. Navigation is underrated in importance and if you keep in mind that when you visit a site you like to be able to get around with ease you'll easily master the concept of standard navigation.
Content makes conversions. Good content will excuse a lot of flaws in design, so if your design is lacking make up for it with great content and hopefully your visitors will stay long enough to read what you have to say.
If worse comes to worse and you follow my guidelines by having a consistent, easily navigable site, with good content and still your site doesn't convert, don't be afraid to get a professional designers' opinion about what your lacking, cause this article is just the bare buttons as far as design is concerned.
