Note: The weight of a DE is comparitive. "Heavy" DEs (such as GNOME) can still be swift on lower spec machines.

GNOME

• Based on Shell Toolkit and GTK4 (with libadwaita)
• Wayland only
• Heavy
• Slightly similar to macOS' UI/UX, but really in a class of its own
• ~~Not particularly customisable~~ Can be customised heavily, but the settings aren't exposed and the devs don't like it much.

KDE Plasma

• Based on Qt6 and QML (with its own frameworks)
• Wayland only (usually)
• Heavy
• Has a lot of dependencies
• Very Windows-y out of the box; but can easily be modified to replicate any other UI/UX

KDE Liquid

• Based on QtQuick
• X11 only (as far as I know)
• Midweight
• No fancy effects
• Not usually packaged, but available on Arch
• Basically just KDE Plasma, but using a slightly different widget toolkit

Xfce

• Based on GTK2/3 (originally XForms)
• X11 by default, but everything except Xfwm supports Wayland (Xfwl is almost done)
• Light
• Generally looks like itself, but some Linux distros have it looking more like Windows

LXQt

• Based on Qt5/6
• X11 by default, but you can switch Openbox for KWin or LabWC in the settings
• Light
• The result of LXDE and Razor-qt merging
• Layout is similar to older versions of Windows, but this can be changed

LXDE

• Based on GTK2 (I believe a GTK3 port may exist)
• X11 only
• Very light

MATE

• Based on GTK2
• X11 only, but it's almost Wayland-ready
• Midweight
• Comparable to Xfce
• Unique 2-bar layout, but can be transformed
• A fork of GNOME 2

Cinnamon

• Based on GTK3/4 (with XApp frameworks)
• X11 by default, with experimental Wayland support
• Midweight
• Windows-esque layout
• Created as a spiritual successor to GNOME 2
• Forked from GNOME 3

Budgie

• Based on GTK3/4
• Wayland only
• Midweight
• Unique layout
• Also created as a spiritual successor to GNOME 2

deepin

• I know basically nothing about this other than the fact it's Chinese
• Looks pretty

Trinity

• Based on TQt3
• X11 only
• Lightweight (these days)
• Similar layout to Windows; actually an old KDE layout
• Forked from KDE 3
• Maintains its own forks of Qt (called TQt), KHTML, and the KDE applications
• Still works with older themes and software, such as QtCurve (which is nice)

Enlightenment

• Based on EFL
• X11 by default, with experimental Wayland support
• Lightweight, despite fancy effects and animations
• Often considered a WM, rather than a DE, but it has its own suite of applications so it's a DE
• Unique layout

COSMIC

• Based on iced
• Wayland by default
• Unsure of weight
• Maintained by System76 (the Pop!_OS people)
• Layout similar to GNOME
• Still quite new

Lumina

• Based on Qt5
• X11 by default
• Quite popular among FreeBSD users

Pantheon

• Based on GTK3/4 and Granite
• Wayland by default (soon to be Wayland-only)
• Midweight
• Akin to macOS
• Used in elementary OS

CDE

• Based on Motif
• X11 only
• Lightweight
• Ancient software, used in many Unices (e.g. AIX, Solaris, Tru64, etc.) and other OSes (e.g. VMS) back in the day

FVWM-Crystal

• Not based on any particular toolkit (a fair bit of raw XCB/Xlib, I imagine)
• X11 only
• Very lightweight
• Quite old, so your mileage may vary when using it with newer software
• Integrates with several music players, including Audacious and Quod Libet
• Arguably more of a shell for FVWM