Examples
Blind people access information on a Web page in a linear manner, via text-to-speech translation. The software tries to extract the text from HTML code. When the software meets a ‹table› tag, it is interpreted like an Excel document with columns and rows. But designers frequently use empty invisible tables for layout purposes, so there is nothing to read in most of the cells.
Another reason not to use the ‹table› tag for the layout is the need some users have to visually reformat a page -- for a person with low vision for example. Such a person uses software that changes the Web page layout by eliminating its CSS or applying a user defined CSS.
There are common misuses of the structure and presentation tags. Sometimes we use ‹H1›attributes to increase a font and ‹B› to make title more visible. But the text-to-speech software reads ‹H1› as a most important header, and ‹H2› as a subheader. On the other hand, ‹B› doesn't carry any structural meaning to the software.
People with colour blindness have difficulties distinguishing between red and green, or between yellow and blue. So, when you design error situations in forms, you should use some error-looking icon or picture in addition to a red colour.
And finally, people with cognitive, neurological, and learning disabilities will benefit from simple, usable, and clean Web sites.
Conclusion
We are a generation of computer geeks. But as we slowly become older people, we should consider our own human frailties. When we decide today to deal with Web accessibility issues, we are actually preparing the Internet for ourselves in the not-so-distant future.







Talkback
Just wanted to clarify - in your list of 12 tips you say:
"Use text presentation via ‹B›, ‹CENTRE›, etc."
I'm assuming you really meant to say something along the lines of "Do not determine text presentation via <B>, <CENTER>, etc..." as those tags:
1. are strictly presentation based, and text formatting and visuals should be done with CSS, and,
2. they provide no semantic logic or structure to the document.