This was the feature that sold me on Mac OS X in 2003 or so. Having an operating system where I could get the dictionary definition of any word in any input field, or web browser, or piece of UI just by mousing-over it and hitting ctrl+command+D
I was a Windows user at the time and the _consistency_ of this dictionary lookup throughout the UI just blew me away
I was a Windows user at the time and the _consistency_ of this dictionary lookup throughout the UI just blew me away