If you let them use Aether Vial to generate mana, they can start producing things that get in the way faster than you can answer them
It can take out three of your opponents monsters right off the bat, which can be a real game changer, and it cant be targeted by any attacks, Spells, or Traps
At the beginning of combat on your turn, put a +1/+1 counter on target creature
Black gets a lot of the 'evil' monsters, with a couple extra ghouls and sorceresses after the break
Over the weekend, Thrun, the Last Troll was revealed as the Goblingram Secret Lair bonus card
Maybe it gets landfall triggers, maybe it has uses for a large amount of mana, maybe it has cards that care about the number of lands in play