You can use your floating_modifier to resize and move tiled windows
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.