On its own, it's weak. It's a little bit of damage. But when combo'd, it becomes absolutely devastating to a majority of the decks the average player will face. It's hard to prevent, hard to predict, and terrifyingly versatile.
Throw it out with a Starving Buzzard on the field, and it becomes a card drawing machine.
Throw it out with any +attack minion (Dire Wolf, Alpha Wolf, Beast Companion) and those 1/1 hounds double in strength, if not more depending on what they have for support.
Throw out a minion that gets buffed from friendly (or any minion) dying, and their suicide charge leaves it alone on the field and a drooling rage machine. (Scavenging Hyena, anyone?)
It's an AOE: 1 or 2 damage to everything your opponent has placed.
It's focused fire: 3 of them can essentially become a 2 mana fireball. Or a more precise Arcane Missile. Take out minions or go straight for the face.
It's instant: you can use them the same turn they're played. Heck, blow them all up, play a second card and do it all over again. (Because after the first wave gives you 3+ cards draw, of course the 2nd card is in there. Naturally.)
It's obscenely cheap for the utility, flexibility, convenience, synergy, and immediacy.
A well-played hunter can punish you no matter what you play, as has been pointed out many times before. Stay smart, play one card? Risk losing it to Kill Command or Deadly Shot. Get ambitious, play 3 cards? Here come the dogs of hell....
And that is why I dread Unleash the Hounds. Because it syncs up with too many other useful cards and is almost gamebreakingly versatile for what it does. I actually miss the old version....