Yes, it is a little facetious to say these mistakes will be made in the year 2015, but it's human nature to repeat your mistakes over and over.
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1. Believing People Care About You And Your Website:
Let’s get this straight: NOBODY cares about your site
Really. What visitors care about is solving their problems.
2.A Man From Mars Can't Figure Out What Your Website Is About In Less Than Four Seconds
You should be able to look at the home page of any site and figure out what the site is about in less than four seconds. If you can't, the site is a failure.
3. Contrast, Dammit
Of all the mistakes, this one is the most mystifying. How is it possible not to notice you can't read the text on a page?
4. Using Web Design Elements That Get In The Way Of The Sale.
Always remember the golden rule of doing business on the Web: "Don't do anything that gets in the way of the sale"
5. Thinking your website is your marketing strategy.
Your website is not your marketing strategy. Your website is PART of your marketing strategy.
6. Navigational Failure
All web navigation must answer these questions:
Where am I?
Where have I been?
Where can I go next?
Where's the Home Page?
Where's the Home Home Page?
Navigation must be simple and consistent.
7. Forgetting The Purpose Of Text
After ten years, you'd think web designers would understand how to use text, but they don't.
Text is Text. Don't use graphics or Flash for text. The first reason is it increases the size of the page; the second reason is it isn't search engine friendly; the third reason is the graphics are often of poor quality and, fourth, mistakes are hard to correct
And, yes, don't use small text, we're all getting older.
8. Too Much Material On One Page
Yes, it's called a web page, but that doesn't mean you have to cram all your material on one page.
It's very easy to keep adding material to your home page until it gets out of control.
With so much content vying for attention, it’s difficult for the eyes to find the focal point.
People get confused and they leave.
A long web page means you have failed to organize your site properly.
9. Misusing Flash
You have to watch a boring, soundless, twenty second flash introduction with no option to skip it. If you're still around when the content loads, the pain doesn't stop. There is a lovely eight or ten second delay between when you click one of the navigation options and when the content actually arrives.
The second problem is JS bloats up your page.