My first point would obviously be that a page should not be designed, but as things go, web page owners make some really stupid decisions when it comes to their website's appearance. I won't be writing this article all at once, but as it comes to me.
I've only really heard one remotely reasonable argument for this: it is easier and less strenuous to read text that does not stretch very far. While this is a fair point, it is forced upon users when it may not be welcome. If I struggle to read something that stretches far across, I will deal with it myself. An ironic feature of fixed width pages is that many (especially blogs) will then not tolerate a smaller width. While is it widely considered stupid not to use xx% values when defining the width of anything, it is still done a lot. The latter there also has its flaws: the size of the browser frame will generally be that at which the user is comfortable reading text along. Defining a xx% merely wastes screen space, and frustrates the user because the page forces its design.
Fixed width is an example of designing a page, which forces features upon the user. It is not a convenience, because not all web pages are fixed width (for example, this one), and so the user would inevitably choose a browser frame size that is suitable for themselves.
I do not mind a page informing me that I am at somesite.org > Writings > Article I am viewing, but I do mind when it wastes a good fifth or quarter of the page with a banner which I have inevitably already seen, yelling at me that I am reading the ONE AND ONLY worsesite.org. It's all about the content, not the site. It is deeply frustrating when content is seemingly the secondary feature of a page (or tertiary, etc. when you add crap like advertisements, sidebars and the like). But what do I know? It's all about the hits, right?.
I will explain some other things that annoy me some other time...