Naturally, this makes you vulnerable to Wear/Tear, Meltdown, and Wrath of the Skies, all very popular cards in Modern
3 Celestine, The Living Saint - Forces Of The Imperium Despite the wings, halo, and a massive golden sword, Celestine, the Living Saint is a powerful human commander with tons of creative interactions with lifegain mechanics
The super-cheap planeswalkers will come down in price, but much like the Elementals from MH2, I expect them to get high again if they start getting playset-popular
This is a special ability - which could be a static effect (Jinx's Loose Cannon legend draws you an extra card if you start the turn with one or less) or an activated ability (Darius's legend taps for 'mana' if you've already played a card that turn)