We've lost some common features:
* Undo & redo
* Help files & context sensitive F1
* Hints on mouse hover
* Keyboard shortcuts & shortcut customisation
* Main menus
* Files & directories
* ESC to close/back
* Drag n drop
Revelation features when they first became common. Now mostly gone on mobile and websites.
And knowing how to get the version of an application (which used to be in Help -> About).
Also having the cursor show up in the proper place on a new form and having tab order work.
Oh yeah.
Keyboard focus and focus indication on browsers is fundamentally broken.
We often notice when focus is buggy. But there are many many hidden/unobvious edge cases.
Previously I created a fix for the worst focusing issues on browsers by creating a fully custom framework (before modern HTML help with popovers/dialogs/overlays/viewports/anchors). No other framework at the time managed Internet Explorer's quirks very well (we served businesses who mostly used that browser).