Keyboard shortcut for Downloads folder

One option would be to create an Automator service and assign it a shortcut in the Keyboard preferences.

tell application "Finder"
    reopen
    activate
    set p to (system attribute "HOME") & "/Downloads/"
    set target of Finder window 1 to POSIX file p
end tell

You could also save the script to ~/Library/Scripts/Applications/Finder/ with AppleScript Editor and use FastScripts to assign the shortcut.

I suggest that you create a custom Finder shortcut in System Preferences → Keyboard → Shortcuts → App Shortcuts → +.

This way you can open packages with ⌥ as a modifier key.

• ⌘+O to open folders
• ⌥+⌘+O to open packages