Morbid -- Gravetiller Wurm enters the battlefield with four +1/+1 counters on it if a creature died this turn.
Six mana for a potential 8/8 trampler. Instead of stating the obvious about morbid versus non-morbid versions of Gravetiller Wurm, I think it's more important to judge this card against other large beaters in Innvasion block that it's likely to do battle with in limited. And the two other uncommon fatties to think about are Grizzled Outcasts and Skaab Goliath.
A morbid enabled Gravetiller Wurm completely trumps Grizzled Outcasts. It also finds itself in a stalemate with the giliath. As you can see, being an 8/8 (as opposed to a 7/7 or 9/9) is very relevant. This is just one of the things you need to keep in mind if you pass a goliath during a draft.
Step 1: Enable morbid.
Step 2: Cast Gravetiller Wurm.
Step 3: Beat face.
The recipe for Gravetiller Wurm decks is pretty straight forward. Morbid is the name of the game and to enable it you either need creatures that like to die --like Doomed Traveler, Gravecrawler, or Strangleroot Geist -- or include a ton of spot removal. Either way, morbid becomes enabled and the wurm does its job.