Mirage introduced two new abilities to the world of Magic: Phasing and Flanking.
Phasing: Now you see it; now you don't! With Phasing, permanents spend only half their time in play, allowing some pretty big discounts on mana cost.
Flanking: Some creatures are just better at attacking than others. And creatures with Flanking are so good, the blockers can go down even before any damage is dealt!