Forcing Burn and Draw. With no and no dual land there is nothing to build toward. If black is open, Goblin Army (57.6% over 85,459 games) is the better deck.
Too few creatures. This deck's and card draw are spells, so a pile of good blue and red cards easily ends up short on bodies. Aim for 15–17 creatures. In pack 3, take Long Lake Nuisance, Patient Instructor or Dori, Bearer of Friends over a third or card-draw spell.
Drafting other decks' synergy cards. Lakeshore Apothecary (55.6% here, 58.7% in Draw Two) and Bilbo Baggins, Burglar are Draw Two cards, and Goblin Plate Mail (53.3% here, 58.1% in Goblin Army) is a Goblin Army card. They are fine here, but don't pick them over removal.
Holding up counterspell mana every turn. Thranduil's Decree does nothing if the opponent doesn't cast into it, and Confusticate and Bebother then only draws two and discards one. Meanwhile you fall behind. Play your creatures and removal on your own turn first.
Casting the creature half of an card from your hand. Smaug, the Great Calamity cast from hand throws Spew Flame away. Cast Spew Flame first and you get both halves.
Trusting a card's win rate in this deck too much. Misty Mountains Raider and Ragged Short Spear have a win rate here from only 810 games, the fewest that get one at all, so a 2-point gap there can be luck.
Cardsplain