At level 5 you get Fireball.
If I remember one thing about my time with Advanced Dungeons & Dragons 2nd Edition, it's that at level 5 you get Fireball. Also Lightning Bolt and Dispel Magic.
In previous editions of Dungeons & Dragons there's a sweet spot. It starts at level 5 and continues on to about level 9. Your hit points are finally high enough that you're unlikely to die from a single unlucky hit. You've got some magic items in your pack. Your magic-users and clerics have evolved from dress-wearing weaklings into fire-throwing killbots and you're finally getting to fight something other than goblins and kobolds.
Later, the shine wore off. Both enemies and players gained access to a range of one-shot-kill attacks, while high level healing spells trivialised death into a revolving door. Non-casting classes became overshadowed by their godlike dress-wearing allies and on the rare occasions when no-one busted out the death rays combats would devolve into an attrition-based slog.
One of the explicit goals of 4th Edition was to take the 5-9 sweet spot and extend it out over the entire game. Largely, it seems to have worked. Almost every level in the game is pretty fun. Except, ironically, for levels 4 to 6. It's an unfortunate aspect of Thunderspire Labyrinth that it lines up its worst content with the dullest point of the Heroic Tier.
How are levels 4, 5 and 6 the weak point? Like so.
New characters are shiny. Usually they're a class, race, or build that you've never played before. They have a range of new mechanics and a new feel to them. You get at least two at-wills, an encounter power, and a daily. You're in a new campaign with new house rules, a new environment to explore, and new party members to come to grips with. At level 2 you get a utility power and at level 3 you get an encounter power, which you can use in each and every encounter if you're so inclined.
Level 4 is when it goes south. You get no new powers at all. Instead you get a couple of stat-ups and a feat. At level 5, you get a daily - something you'll probably only use once a session. And then at level 6 you get another utility - which is, again, most often a daily.
This means at exactly the point where the shine is coming off your initial new-character love affair, the game stops handing out new toys. Whatever it is that you're doing to kill enemies at level 3 is pretty much what you'll be doing in every fight until level 7.
That's a long, hard slog. With a new Player's Handbook or Power supplement coming out every other month, it's tough not to think about re-rolling during the winter levels.
The solution is on the DM side. Level 4 to 6 is the time to take off the training wheels. Encounters should be harder, leaner, and just plain meaner. Loot should be more plentiful and players should be living or dieing on the basis of their initiative and cunning. It's time to present bizarre monsters, baffling traps and your most intricate dungeon setpieces.
It's Thunderspire's tragedy to throttle back on all those things just when it should be stepping up the pace. It takes players for a leisurely stroll through a hobgoblin hideout when it wants to be pitting them in a desperate struggle. It's got a series of knock-down fights against dwarves where it should have an intricate diplomatic contest. Players accustomed to Keep's focus on minions and terrain will find not a single minion awaiting them in the Labyrinth and only the driest of tactical layouts on offer. Where Keep was showing off a different 4th Edition mechanic in every combat, Thunderspire is happy to confine itself to the most basic actions contemplated by the Player's Handbook.
Thunderspire eventually takes off the gloves. The module's third mini-dungeon, the Well of Demons, busts loose with a fiesta of classic dungeoneering, including traps, puzzles, skill challenges, and cursed artefacts. The "Proving Grounds" section of the Well is one of the best set-pieces ever presented in a published adventure.
But by the time players reach this point, it may well be too late. Over the early parts of the module, the shine well and truly wears off 4th Edition, and the level 4-6 Sour Spot will cause the majority of groups to fall to ennui before getting the chance to actually experience the final third of the Heroic tier.
 Would it have been so problematic to move the Level 7 Encounter to Level 5, the 5 Daily to 6, and the 6 Utility to 7? It creates a more natural flow over the early levels of the game and puts less pressure on the DM to keep things interesting over this critical period.
 Races are an underused element of 4th Edition. Would the addition of a Level 5 Racial Encounter Power, gained on top of the existing Daily, help fix 4th Edition's Sour Spot?