It could very well be what makes me want to buy the vampire deck over all others even if I know it is a cheap card and I pulled a bunch when it was in standard
The best Angel commander in MTG is Giada, Font of Hope because she provides mana acceleration and permanent +1/+1 counters for Angels
Key also created all-or-nothing situations, because you can strand the Key without anything worth untapping
Some cards that like other creatures boasting include Frenzies Raider, who gets a +1/+1 counter whenever a creature boasts, Dragonkin Berserker, who reduced the boast cost for each dragon you control (before making even more dragons with his own Boast ability), and Birgi, God of Storytelling, which allows each creature to boast twice instead of just once each turn
I suspect Goblin King cost another mana because there were twice as many Goblins in the first set as Merfolk, thus making him more effective