Lets look at the combos, starting with the two-card combos
Any time you have priority during a main phase of your turn and the stack is empty, you may pay the mana cost of a locked door (also called its "unlock cost")
Cards like Waterbender's Restoration, Extravagant Replication and Thassa, Deep-Dwelling repeatedly flickering your Aether Channeler and Riftwing Cloudskate gives you an incredible advantage because your opponents have to recast the same creature over and over (this is trickier in our era of powerful enters abilities, but still works)
It was so severe that some collectors ordered huge numbers of Happy Meals purely for the cards, to the extent that McDonald's stores had to put maximum purchase limits in place, while the head office requested potential customers "think twice before placing excessive Happy Meal orders during this promotion"
You want draw triggers
Junk, on the other hand, I think is awesome