- The lack of overhead required by security and 'features' like querying an internet search when you click the start menu or showing advertising in the calculator or better memory management in general, meant that overall UI response in that era used to be much _MUCH_ faster than it is today. Once upon a time you could operate your O/S with the speed of a Starcraft tournament winner; this simply is not possible any more.
- You could 'queue' commands - clicking the close button on an program and then (before it had finished closing) clicking the minimise button would minimise the program behind it immediately after the closing process finished. In this way you could chain/queue commands rather than being forced to wait for the OS to update between each step.
- Enter and Spacebar did different things. If a prompt had two buttons, one would be outlined with a thick black line that would respond to [Enter], and one would be outlined in a dotted line that would respond to [spacebar]. This is still the case sometimes but is far from ubiquitous.
- The top-left corner of the program was reserved for a 'system' menu used to move/resize the window, or quickly exit with a double-click. Though still used by some MS programs today like Explorer, its usefulness is lessened if not all programs utilise it.
- Don't even get me started on keyboard shortcuts.
These kinds of universally-accepted and _useful_ power-user-oriented design principles are almost absent from UX as it is implemented today.