The internet is a great thing and seemingly inundated with noisy advice on what you should do for your website. I'm no different in that I have clear opinions (http://www.anadguy.com/2009/01/01/wordpress-what-is-it-how-do-you-use-it-why/) on what you should do with your website.
But this piece is about the single most important bit of advice that will keep your website from looking amateurish or even stupid.
Limited Column Widths Limiting a block of text to a width that amounts to approximately 15 to 20 words makes for a comfortable reading experience. Unfortunately, too many websites use 100% as the width for columns of text, sometimes using only margins and padding to provide whitespace around that text. If a website's overall layout is also set to 100%, that means that the layout will expand and fill the horizontal space allowed by a user's browser window. The larger the window the wider the text.
If you have a small monitor or laptop, your browser may keep that website constrained to a manageable width. But if you have a high-resolution set on your laptop, making details finer and smaller and website layouts smaller too, or if you have a large flat-screen monitor, it's very likely that you'll have a website that stretches and fills the entire width allowed by that screen.
Reading the main text on a website like that, you'll notice that it becomes very difficult to keep track of where a line of text ends on the far right and where it picks up on the far left.
In olden days, pre-internet and web design, this was called "bad typography." Wikipedia is an example of a great site with bad typography for this specific reason.
Good typography has one overriding purpose, legibility. Really good typography finds a sweet spot where beauty and functionality coexist happily. So, really good typography not only is legible, it's esthetically pleasing and, moreover, it makes you want to read it.
Can you imagine someone wanting to read your website?
If you want to guarantee that your website is legible, doesn't look amateurish (at least as far as the typography is concerned) and has a strong possibility of being persuasively readable, then make sure that your web designer knows this simple typographical rule. Columns of body copy, the text that comprises the main content of a page, should be limited or constrained in their width.
Some website designs are made to be able to "zoom" as the user increases the size of the font, if they find a larger font size easier to read. That way the layout doesn't break as the text gets larger. Other designs are static and don't scale the type. Either way, the main body of text should not be so wide that the reader loses their way as they scan from left to right and back again.
If you're developing a website for your business or blog, make sure it treats its readers kindly, starting with Limited Width Columns.
Action Steps
The best contacts and resources to help you get it done
Great Site, Bad Typography
Wikipedia is a great website, but its main body of text is set to fit 100% of whatever the width becomes when it's viewed in a browser window. If the window is narrow, no problem. But if it's wide, the text becomes very difficult to read. Try it for yourself and see how annoying that can be.
I recommend: http://en.wikipedia.org/wiki/Wordpress