Too few creatures. This deck's is so good that beginners end up with a pile of kill spells and nothing to attack with. Aim for 15–17 creatures, five or six of them , and count spells (Tidings of War, Rage into the Valley, Goblin Plate Mail) as half a creature, because they only make a new body when you have no .
Drafting by name or rarity. Bolg of the North (the gold uncommon made for this deck), Fearsome Goblin Pair, Snowslope Hunter and the rare Azog, Moria's Ruin all read well, but each wins less often here than most of the deck's plain commons. Check a card's win rate in Goblin Army, not its rarity or a pre-release review.
Mixing up the Smaugs. Smaug the Magnificent is the 4-mana mythic . Smaug, the Great Calamity // Spew Flame is a 7-mana common Dragon whose 5-mana is removal. The 5-mana gold (black-red) rare is Smaug, Wicked Worm. Desolation of Smaug and Smaug's Fury are not Dragons at all. Read the cost and rarity before you pick.
Ignoring the . Several of the deck's commons are (Dori, Bearer of Friends; Gollum, Silent Slinker; Smaug, the Great Calamity), and you can't keep two with the same name on the battlefield. Two copies in the deck is fine, but a third is worth less than a different card.
Putting everything on one Army. Every amass card stacks on the same , so one removal spell, or an opposing Crude Bent Blade when the Army is your only creature, wipes out several cards of work. Keep a small creature next to it, and block with the Army when the damage it stops is worth more than the counters you lose.
Counting on triggers against . Pinecone Strike, Gnashing of Teeth's -5/-5 mode and Head of the Hunt exile a creature instead of letting it die, so Fearsome Goblin Pair, Rhovanion Rampager and Front Porch Sentries get nothing from them.
Cardsplain