I like the new reminders app in 10.8, but I was disappointed to find that there is no way to create new reminders from outside the app (like, e.g., OmniFocus, which has a quick entry window that can be summoned with a keyboard shortcut). I created a service in Automator that allows such entry, albeit with limited functionality.
Create a new service in Automator and choose Service receives no input in any application at the top of the window. Next, in the Text library, choose Ask for Text and drag it to the right-hand part of the Automator window. Enter a prompt, such as "Enter a Reminder." Finally, from the Mail library, choose New Reminders Item and drag that below the Ask for Text item.
Save the service, and use the Services tab in the Keyboard preference pane of System Preferences to add a shortcut that will let you launch this from within any application.
This only allows you to set new reminders in a single list, with a single priority, and with no due date (unless you want everything due on the same day), but it's better than nothing. One more caveat: it's slightly unreliable, because some programs interfere with the Services menu or don't accept Services.
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