Fast search
Using CTRL + K (or CMD on Mac) you can search servers, channels or users. Add a * to exclude channels & users and only show servers.
I assume you know this, but for MAC
CTRL = CMD & ALT = OPTION
Back/forward in history
This works for me on Mac, not sure about windows. To zap between last server (back & forward): CMD + [ or ]
Select messages easily
Press TAB, then select your message with UP/DOWN.
Reply with R or react (emoji) with +
Toggle between channels with mentions
Press CTRL + SHIFT + ALT + ARROW UP/DOWN to zap between channels with unread mentions.
Mark as read
- Mark channel as read ESC
- Mark server as read SHIFT + ESC
Easy Delete/Actions
Hold SHIFT and hover a message to show additional actions.
All shortcuts: MAC 
All shortcuts: Windows 
Use timestamps for dates and times
Insert a date/time here and copy the result in Discord for dynamic timestamps that automatically adjust to the timezone of users.