Missing vertical taskbar is probably the most egregious omission, but it's not so much they took it away as it is they created versions of a number of Windows Explorer components in a higher-level technology without implementing half the features and shipping it with 50x the number of bugs. I at least weekly (and often daily) run into issues with taskbar icons overlapping one another, menus not coming up when clicked, the tray icons breaking, etc.
Same story with navigating the file system--the new implementation has a multitude of issues, including getting into a state where clicking files to select them only works below a certain invisible horizontal line in the window, windows not refreshing when files have been added/removed, trying to rename a file you just copied being an exercise in frustration with the view refreshing and exiting the rename state 5 - 10 seconds after the copy, the address bar breaking in about a dozen different ways... it's really frustrating software that's a full few tiers down from the quality standard set by Windows 10 and previous versions.
It's gotten slightly better since the initial Windows 11 release, but it still feels like pre-release quality software. I was hoping they'd get it up to release quality and add the important features back by the sunset of Windows 10, but it looks like Microsoft really doesn't care about the quality of the experience of using their UI.
If it were only missing the vertical taskbar as a design decision that would be one thing, but instead it's the very obvious tip of an iceberg of lack of user focus, care, quality, resourcing, and skill. They don't add it back because they know in their current state they're not going to do it well, and the money's in dreaming up new ways of force-feeding trash "news" and promotions anyway, not in helping you get things done and providing a well-functioning tool and bicycle for the mind. What if someone put the taskbar on the left side of the screen, it interfered with them seeing the clickbait brainrot of the widgets "feature", and Microsoft didn't get its average $.0003 for each interaction?