Gmail is the only Google service that disables keyboard shortcuts by default. They're for power users and some people could accidentally trigger them, especially when you consider that most Gmail shortcuts don't use modifiers.
Google has recently added some new shortcuts that are enabled by default. The old shortcuts are still disabled by default, but you can quickly enable them by pressing ? in Gmail and clicking "Enable" next to "The following keyboard shortcuts are currently disabled".
I've already mentioned some of them, but here is the full list of shortcuts that are enabled by default (you can't disable them):
- arrow keys: you can use the up/down arrows to navigate in a list of conversations and press Enter to select one of them. Gmail automatically loads the previous/next page of conversations. Use the left arrow key and then the up/down arrow keys to move your cursor to a different label or the compose button.
- n/p: if you open a conversation with multiple messages, use the n/p keys to go to the next or previous message. Press Enter to load that message if it's collapsed.
- Ctrl+Enter: use this shortcut in the compose window to send a message.
- Shift+Esc: focus the main window.
- Esc: focus the latest chat or compose window.
- Ctrl+. or Ctrl+,: move the cursor to the next/previous chat or compose window, or to the main window.
- Ctrl+Shift+c: add CC recipients in the compose window.
- Ctrl+Shift+b: add BCC recipients in the compose window.
- Ctrl+Shift+f: change the From email address in the compose window.
- A long list of shortcuts for formatting text when you compose a message.
Ctrl+Shift+2 - insert emoticon
Ctrl+Shift+7 - numbered list
Ctrl+Shift+8 - bulleted list
Ctrl+Shift+9 - quote text
Ctrl+[ - indent less
Ctrl+] - indent more
Ctrl+Shift+l - align left
Ctrl+Shift+e - align center
Ctrl+Shift+r - align right
Some of them were already available in the old compose interface:
Ctrl+b - bold text
Ctrl+i - italic text
Ctrl+u - underline text
Ctrl+k - insert link
Obviously, Mac users should replace Ctrl with ⌘.