Hi Stuart!
There is a huge difference even between these two browsers - IE and NS. They have different JavaScript API, they differently show tables, differntly support CSS and many other "big difference" things.
People like to say that there are no two exactly identical computers in the world

That's why compatibility is a BIG issue.
Monitor colors - how many colors does a visitor have? 16, 256, 16 bit, 32 bit?
Monitor resolutution - how big is it? 640x480 or 1600x1200 or even bigger?
Browser - the main thing and may be the main problem. Is it IE, or NS, or AOL, or Opera, or any other?
Browser version - what browser version is it, by the way? NS 4 and NS 6 are two different browsers.
Operating system - is it Win, or Mac, or Linux, or any other? Sometimes same browsers with same versions has differences on fifferent platforms.
JavaScript - does a browser support JavaScript? Is JavaScript enabled on clients computer? Is it JavaScript or JScript?
Cookies - does a browser support it and is it enabled?
CSS - how browser supports CSS? What styles exactly does it support? How this styles should be implemented? Browsers (mostly old ones) sometimes complitely different support some styles. Sometimes for one browser you may call style with class attribute, but for another browser you need to use some container tag such as div or span for style to work.
And there is a number of other things which are important for compatibility/accessibility also.
- What type of browser client uses? Regular one or text-only browser, or Braille-browser? Can you imagine that a blind person may view your site - what have you done for your site to be read and understood by such people?
- How good/bad looks the site without images? People may use text-only browser or simply surf the Net without images enabled in their browsers.
- How good/fast is client's connection and how long does it take for site to be totally loaded? Will a client with a slow connection wait?
- How good/fast is client's computer? Many Flash presentations look bad on slow computers.
- and so on...
I agree may be it's too much but these things (and many others) are the compatibility issue.
Good Luck!
