Most of this argumentation covers issues of web site navigability, some of it comes from slavish devotion to how other media forms set up navigability, and some comes from assumptions about people's computers and user interface expectations.
For example, the vast majority of web sites, like the vast majority of browsers and computer applications, follow what's called ''Fitts Laws'' - this is the discovery, made in the Xerox and Apple Usability Labs in the early 1980s, that most people who are unfamiliar with the mouse will find it easier to locate a control bar if they move their mouse to the top of the screen or the bottom of the screen, where the cursor stops moving.
Now, these are all well and good - but note the assumption in there: A user who is unfamiliar with the mouse. How many users of the World Wide Web are not going to be proficient with using a mouse by the time they hit their first web site?
There are other untested assumptions. Jakob Nielsen at Sun Microsystems defined a set of Web usability standards in 1994, that got updated to 1996....and which some people still slavishly adhere to. When Nielsen wrote those standards, the average screen resolution was 1024x768, the vast majority of people were still connecting to the Internet with 48k modems, and every possible kilobyte of graphics were removed from web pages to improve performance.
Nielsen also tends to be rather apoplectic that the HTML standard is, first and foremost, a text-based standard. You can readily identify pages designed around Nielsen's teachings: They don't use columns, they're seas of text, and they use graphics apologetically rather than to build an overall site aesthetic.
This isn't to say that it's time to throw those usability standards out; thinking about your web site from the perspective of someone who has a slow Internet connection, or from someone using a very small screen (such as a PDA or iPhone user) is a worthwhile design exercise. Far too many designers assume that their web site will be seen at a specific resolution with a specific set of fonts installed, and a handful assume that that resolution is full width screen at 1920x1200 resolution.
Others get far enough involved in the process that everything in the web site comes through a Flash front end; Flash has its place, but that place usually shouldn't be in user interface design. Likewise, the ability to separate content from presentation layers with Cascading Style Sheets means that you have more flexibility available to you than mid 1990s web site designers ever dreamt of.