h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

The font of a site has to he simple otherwise people can't be bothered to try and read it. The layout should be simple but that doesn't mean it can't use clever little tricks like pop-down menus etc. As long as the navigation and layout is easy to follow then it should be fine.
Colours are best as background unless you have a very simple picture pattern as a background and a text colour over it so that the text shows up!
Pop-up ads are never 'worth it' but for the normal people (i.e. those of us without loads of cash to spend) like me they're often unavoidable. Your own personal pop windows are fine as long they contain relevant content and don't pop up without being asked to.
Large files are useful but shouldn't be used unneccasarily. If a page really needs large pictures or diagrams on then put a warning at the top of the page that they will take a while to load.
Most sites do work on all browsers and it's only the webmasters problem really if they don't work on all because he won't get as many visiters.
Some sites have 'background music' so that there's some nice sound instead of just your computer humming. I think this is up to the webmaster and on most sites you don't HAVE to listen to the music it is optional.
As far as I know the only person who uses a sitemap is a webmaster when he's checking things or editing the site to see what's linked to what otherwise no-one uses it.
There's nothing wrong with Animated GIF's they can be good fun and they're no worse than normal pictures.

The easiest way to make websites user-friendly is first, to make them fast. If it is fast the user can navigate through the pages quickly and see what he wants without getting bored inbetween. You also have to make sure that the user can move to any page at any time. There are plenty of sites wher the only way to navigate back from a page is to press the 'back' button your browser, an absolute pain. A simple navigation bar is all you need, which has links to all the key pages in the website on then your user will not only be able to quickly from page to page but can understand the pages and find what he's looking fast then he'll be happy. As long as the site content is good in the first place!

Ideally, you don't force someone to use a particular font. You influence the relative sizes and whether it has serifs or not, but you shouldn't require a specific font. I've set the fonts in my browser to the combination that best works for me and when websites override that, it can actually make it more difficult to read.