Widgets. Widgets allow you the option of selecting various bits for whatever containers the skin you've selected has, and ordering them in the manner you see fit. Widgets are in the admin part ;)

Custom CSS. Custom CSS per blog or user or both allows you and your bloggers the option of tweaking a skin on a case-by-case basis by uploading a customized bit of styling. Custom CSS is in the admin part ;)

Personally I'm totally into skins NOT being just a dumb style sheet selector. I *like* that I can have completely different presentations for the same exact content - and I don't mean just coloring the text differently. Add or remove the author's name ... or the word count ... or link the author's name to all their posts ... or have a different number of posts on a multi-post page per blog ... or have some blogs go newest to oldest and others oldest to newest. Styles are cute and all that, but the real flexibility comes from code. Skins do style AND code :D