Re: Theme designers: Use CSS classes, not IDs.
Using classes for everything is pretty sloppy. When start hacking up the bbpress theme and come across an ID I know exactly whether or not that element is going to be repeated. When I work with a design that has strickly class based selectors I go crazy.
Something like “class=header” is completely insane. There really should only be one header inside the site and you use an ID to signify that elements usage.
For example when I take a look at /bb-templates/font-page.php and see <table id=”latest”> I already have a good idea of what all the code contained inside that <table> tag will be.
Also if my page gets larger I can put “jump links” to those sections and help my users navigate my site.