Giving your opponent a card to make them attack somebody is too great a disadvantage, even if you get some mana
Finding the balance where a planeswalker can be relevant in a game without being overpowered is hard to get right on the first try
5 Manaform Hellkite Weve got another Dragon who fits in Spellslinger decks, so thats nice
Doctor Doom, Unrivaled You rarely see creatures that just tap to draw cards, which makes Doctor Doom, Unrivaled a little interesting
You always get a rare or mythic rare that is a card with the showcase fang treatment, a showcase eternal night legend, a borderless dual land, a borderless planeswalker card, or a Dracula series card
Its fair to see that most people have found at least one of these to be to their tastes now, and the most recent releases - basically all of the ones named above - have ranged in quality from being very good to exceptional, making the concept a pretty resounding success