Well, validation on its own isn't all that important to me. Coding to W3C standards definitely is.

If everybody coded their pages to some form of standards, W3C or not, and browser vendors made their software support standards correctly and in full, then we'd have a definitive way of working with markup. That is to say, we should be able to go out, code a page, and it will work in every browser out there, thus enabling access to as wide an audience as software permits. So for example, a page could be coded that works in IE for Windows users, Firefox for say Linux users, Lynx for visually-impaired users and so forth.

Of course there are other benefits of coding valid markup. You can have those buttons on your pages that say that it complies to W3C standards, which is always nice. It also promotes a more professional view of yourself, your company, or both. It means that other coders should be able to grab a copy of the source code and find a quick way to change something (to an extent). The list goes on.

The same applies to all web languages: HTML, XHTML, XML, CSS, SVG, RDF etc.

With regards to browser-specific code, I generally try and avoid this. If you want a feature implemented, you might as well implement it for your whole audience, not just a percentage of your site's users. Understandably, some people would rather use CSS3 properties for rounded corners than images because of the file size, HTTP requests and rendering times. But if only 10% of your users reap the benefits, then images it would be, for me anyway!