It's clean because it does not include any file extensions or query strings. Clean does not equal semantic, you are correct. You can of course change the name of any page (e.g. "about").

Our new sites are all deployed with pathauto, which allows you to generate page names automatically based on their type, title, user, etc.

How many users actually care what the url is? I've read a lot of conflicting information about this. A lot of people really do click around, make book marks, and they don't look at URL's. Why is it such a big deal if a url have a bunch of query string data or file extensions in it? Is a clean semantic URL really going to improve the quality of life?

Query string data will lower your search engine ranking and/or make it difficult for search engines to crawl. And lacks semantic information along the path like monkey/eats/bananas that result in higher search ranking.

File extensions are an interesting one. I believe that something like monkey.php -- or even -- monkey.html -- adds little value and makes it difficult to migrate between underlying systems and keep the same URLs. So, if you can, abstract away the file endings from day one, and it will be easy to transparently swap systems underneath without changing any URLs.

For bookmarks, URLs don't really matter, but certainly the titles of pages do. If your site all starts with MyDomain - Title of Page, then all the bookmarks will start with MyDomain, making it hard to see which bookmark is which. Flip it around, and you have Title of Page - MyDomain, making it much easier to differentiate bookmarks.