Keyboard shortcuts

Pros

Fast. It's short and it cuts.
No visual clutter. It is invisible. Outside the hints in tooltips, or shortcuts summary like in Dropbox
Good coverage. Most actions can be bound to a shortcut.

Hard to remember. It's hard to come with shortcuts that people will remember, especially those you don't use everyday. It's even harder to remember them when they are app-specific, and few shortcuts are universally accepted (Ctrl+C, Ctrl-Z...)
Mediocre discoverability. This can be solved by showing hints in the right places (tooltips) or documenting the list of shortcuts.
Contextual inconsistency. Shortcuts are not visibly tied to a context. It's not always obvious when a shortcut is effective or not, and what it will do. For example, should a blockquote shortcut always insert a blockquote, or toggle the current block to a blockquote?
Conflicts. Custom shortcuts can conflict with existing shortcuts (e.g. with browser shortcuts).