• Sway is a Wayland compositor (duh)
• Sway handles things like output configuration (rather than something like xrandr)
• Sway supports multiple non-modifier keys when assigning key bindings (see Shortcut handling)
• The default keybindings in sway are different from those in i3. Read your sway config to learn them, or copy over your i3 config to use what you're used to.
• Sway handles your wallpaper
• Sway handles input configuration (man sway-input)
• Sway is documented through man pages, rather than the website
• A few i3 commands only make sense on X11 and aren't present in sway
• You may occasionally run into minor inconsistencies between sway syntax and i3 syntax. Just tweak your commands to be a bit more explicit if necessary
• i3lock forks into the background when run. For swaylock, this behavior has to be explicitly requested via -f
• i3 has support to export a layout and to restore it (or parts of it), sway only has export, no import.
• More stuff, some which are listed in this reddit post.

Not a difference anymore:

• You can use your floating_modifier to resize and move tiled windows (since v4.24)