What are your thoughts on content accessibility?

Accessibility
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Use the clearest and simplest language appropriate for a site’s content.

The key word here is appropriate. The language should be appropriate to the purpose and the intended audience of the website. “Appropriate” encompasses common sense.

Does anybody agree with this or is it just catering to the lowest common denominator?
Is my site really regarded as inaccessible because I have a reasonable command of the English language and can express myself well?
Is this not the equivalent of insisting that authors write books in an easy-to-understand, as opposed to literary (thus enjoyable) style?

You are losing the point of the guidelines. They are not for making most entertaining, or best written, but most accessible website. Follow the guidelines to have the maximum number of people be able to use your site. And yes that does mean designing to the lowest common denominator. However, you get to pick that denominator. Determine your intended audience and design your content to be accessible to them.

Wikipedia: Accessibility is a general term used to describe the degree to which a product, device, service, or environment is accessible by as many people as possible.

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If you use difficult and unusual words that make some users unable to understand what you are saying, then yes, it is inaccessible to them. If those people are part of your ‘target audience’ you have a real problem.

There are many reasons why someone may have difficulties understanding what you write: a cognitive disability (dyslexia, aphasia, etc.), deafness, English as a second/third/… language, and so on. A lot of people seem to read very little, if at all, these days. This lack of practice is detrimental to their reading comprehension. I think that’s why video on the Web is becoming so popular: people are getting increasingly bad at reading.

That depends on what sort of site you have. In many cases a more apposite analogy would be an instruction manual or an informative pamphlet/brochure targeting the general public. If it’s essential that everyone understand the content, you’ll aim for the lowest common denominator.

A literary novel is normally targeting a smaller audience, viz. those who enjoy reading a richer linguistic style.