Deckbuilding around mythic rares means planning your entire strategy with those cards as the centerpiece
RW, Unattach Sunforger: Search your library for a red or white instant card with mana value 4 or less and cast that card without paying its mana cost
Tapping is a core mechanic of Magic: The Gathering, and being able to tap your own creatures can be used as a resource, much like mana or your own life total
Fans will be used to this by now, but at the time it was quiet the innovation that this set (and the next one) would be available in paper at prerelease events prior to its digital launch
As for me, if I see a card that can singlehandedly warp the game, sign me up immediately
Also nifty support for man-lands and cards that turn your lands into creatures, of which green has plenty