100WattWalrus
CMD+SHIFT+B Insert Bulleted List
CMD+SHIFT+O Insert Ordered List
CMD+K Insert Link (standard shortcut throughout Mac apps, like CTRL+K is in Windows)
CMD+SHIFT+T Insert Time Stamp
CMD+SHIFT+X Strikethrough (standard throughout most apps with this style)
CMD+OPT+T Add Tags (SEE NOTE BELOW)
CMD+OPT+P Add People
CMD+OPT+I Add photo (I for Image)
CMD+OPT+R Add Rating
CMD+OPT+A Add Audio
CMD+OPT+F Add File
CMD+OPT+L Add Location
Implemented all those as suggested, except "Add audio" and "Add rating" as they require additional mouse interaction, so a keyboard shortcut didn't feel very useful. One could say the same about the "Add photo" shortcut, but it's probably more commonly used than the other ones and also would require 2 clicks normally.
100WattWalrus An alternative to this — for Mac users anyway — would be to include these features/actions in the menus. Mac users can make custom keyboard shortcuts for anything in the menus.
That was a great idea and that's how I implemented it. Made things a lot easier for me as well.
Are [ & ] commonly used? Don't they require the use of another modifier key (OPT) to be entered?
Also, with "Back" and "Forward" do you mean the buttons left and right of e.g. the date in the detail page or the calendar/timeline pages?

100WattWalrus
CMD+N New entry for current date
CMD+OPT+N New entry from a template (brings up templates to choose from)
Wouldn't those shortcuts only make sense when there's also shortcuts for "Save entry" or the "Back" button, that goes back to the previous page?