Twan Z
In addition to Headers, Footers, Templates and Components:
The option to set up Site Wrappers (and its id and classes) that will wrap around any combination of header+page/template+footer, with its own conditions setting.
<body>
<div id="site-wrapper-x" class="i-am-a-site-wrapper">
<!-- whatever header template meets current conditions -->
<!-- whatever regular template meets current conditions -->
<!-- whatever footer template meets current conditions -->
</div>
</body>
It is debatable how essential this feature really is, but such an outer wrapper has been part of my project setup for years, and judging by multiple comments in the Facebook-group, I'm not alone in this.