Gnome comes with hardly any customization options. For the vast majority of users this probably is a good thing. Personally I feel a lot more comfortable and efficient with a couple of customizations. To work efficiently I need to keep a vast amount of windows (~100 at peak times) open at the same time and switch between them with minimal effort.
Here's my current list of Gnome extensions that make using gnome a lot more tolerable for me. I'm running Gnome 3.38 right now and didn't really look into Gnome 40 yet.
- AlternateTab - because trying to switch though grouped windows steals a lot of time and quite frankly makes me angry. So this is an easy but important UX fix.
- AppIndicator and KStatusNotifierItem Support - IMHO the most important fix all Gnome users need. It adds a status icon area. Some apps I need to use are only accessible through their status icon. (nextcloud, nitrokey app, etc.) I've tried a lot of status icon area extensions over the years. This is the only one which doesn't bug out. Lucky break.
- Audio Switcher - I've got a couple of audio sinks (like external sound cards on my docking station, various bluetooth devices) This make switching bewteen them considerably less annoying. It also add a handy microphone volume indicator, so I can always if my microphone is turned on.
- Current screen only on windows switcher - does exactly what the name sais. This way alt-tab is less confusing for me.
- Frippery Move Clock - It's just easier on my eyes, when the clock is not cluttering up the center of my top bar.
- Impatience - Extension for scaling down animation durations. I'm trying to get some work done here. Watching pretty windows fly across the desktop is fun sometimes, but I'd rather navigate quickly.
- Vitals - So far my favorite system monitor plugin. It's highly customizable and supports monitoring really a lot of things. You could use it to pin your secondary casing fan speed or your wifi chip temperature to gnomes top bar. I've only pinned the most basic things and still numbers it's tracking are 2 clicks away at most. It has proven itself very useful to me time and again.
- Workspace Matrix - I use a lot of workspaces and prefer a vertical layout. This makes it feasible.