Thunderspire Labyrinth has no minions. Not one.
It may have been a deliberate stylistic decision, but I suspect not. I think Thunderspire was being written while the 4E rules were still in flux, and Richard Baker never got the memo that minions were a D&D thing now.
Minions are a new thing to 4th Edition, although the concept's been floating around in games like 7th Sea for over a decade now. "Minion" monsters have the defences and special abilities of a regular monster, but they do fixed damage rather random, and they die as soon as they take damage.
They're a great mechanic. Killing a minion is just as satisfying as killing a real monster - maybe moreso - and they let the DM field enemies on a scale that would have slowed previous editions to a crawl. They let players really feel like they stand head-and-shoulders over the average opponent, and the ineptitude of minions makes "real" monsters even more impressive by comparison.
Minions aren't just a flavour issue. They're built right into the game balance.
Warlocks gain a minor bonus every time they drop an enemy to zero HP. So do Battlerager Fighters. Both these characters are custom-built for removing minions from the battlefield. Wizards, Druids, and any other area of effect class are more effective when there's more minions on the board, as their powers can hit more targets and yield a much higher proportionate reduction in the enemy's damage-per-round. Paladins get better when they get surrounded, and Fighters get a lot of value out of Cleave-ing onto adjacent minions to earn two kills for the price of one.
Rangers, Avengers and Rogues, by contrast, can be significantly hampered by minions, who are able to restrict their mobility at little cost and render irrelevant the large damage totals that a striker could otherwise deliver.
So when Thunderspire forgets to include minions in its encounters, it's a real issue. If you're playing through this module, you're going to find levels 4 through 6 are the era of the striker, with high damage ruling the day, while other classes feel as if there's something fundamentally missing from their game experience. There is - it's minions.
 Wherever possible throughout Thunderspire's combat encounters, remove one regular enemy and replace it with two minions of equivalent level. In large combats, swap two enemies for four minions. You'll find the fights are better paced and more satisfying.
EDIT: It's been suggested in the comments that three or four minions per normal monster is a better ratio, and I agree. Depending on the effectiveness of your characters, you may want to go as high as five or six.
 A real danger throughout Thunderspire (which I'll be talking about eventually) is what MMO players refer to as "adds". Outnumbered monsters routinely flee to adjacent rooms and bring back allies, which can quickly turn a balanced encounter into a party-killer. You can kill two birds with one stone by declaring all non-named adds to be minions and awarding only half XP for their defeat.