Re:

I noticed some places where fern could use some more work (using the latest Firefox).

1. My site name was not displayed, because I have an additional picture as a logo and your code displays either only the logo or only the site name but not both. I think displying both is the most common use case.

I'm not sure I agree. However, you can alter the php if{} statement in page.tpl.php to remove the check and make it do as you wish.

2. The layout is "too fixed": it keeps the space of the left sidebar empty even if you don't use a left sidebar. Usually "fixed width" means only fixed at runtime, or so most fixed width themes work.

Indeed. The 'One True Layout' markup is very fixed indeed when using pixels for column widths. However, if you replace the layout sheet with one which uses % values instead you can get a full-width layout. There's a link to the public tool available in the README.

3. I am curious: why 700px and not 760 or 780 for those with 800x600 monitors?

Because there's an invisible grid in play. Columns are a multiple of 150px (seperated by 25px gutters). Again, you can replace the layout sheet as described in the README.

4. At the default width, fern makes some administration tables unusable. For example, on the theme selection page you can't choose the first three themes because the right sidebar covers the radiobuttons.

Maybe you could add a scrollbar using something like:
#main table {
display:block;
width: 100%;
overflow-x:auto; overflow-y;hidden;
}

This is a suck which applies to pretty much all fixed-width layouts; I've not given it much attention for two reasons. First, because it only 'breaks' like this in admin/. And second (and more usefully) because sections.module allows you to choose another theme entirely for the admin area if you wish to use one.

(Bluemarine just pushes the sidebar outwards. Ugly, but it works.)

Bluemarine is table-based and for that reason gets nul points from the Standards Jury. </eurovision>