Here is something that the Well of Demons does well: building tension.
Without a doubt the best section of Thunderspire Labyrinth is the Proving Grounds. This is the central portion of the Well of Demons, intended to test potential worshippers of Baphomet before allowing them access to the inner sanctum. The Proving Grounds is essentially an extended lock-and-key puzzle consisting of a total of five encounters and culminating in an epic battle with a green dragon.
Thunderspire knows that the Proving Grounds is good. It knows that fighting a dragon on its home turf should be a big thing. And so it finally does what it should have been doing all along: it gives us a little showmanship.
The module introduces us to the Proving Grounds end-first. The very first part of the puzzle that the players see is the conclusion - the massive purpose-built arena in which they'll end up fighting the dragon. Of course, at this stage they don't know about the dragon, and very fact that such a massive and complex area is apparently unoccupied is both ominous and foreboding. There are a number of deep holes, a looping corridor with disturbing grooves in the floor, and other odd features such as glowing pools of liquid and ruined statues; players have another four encounters before they see how it all works and that's plenty of time for their imaginations to hype the area into a killing floor of unmatched ferocity.
The first of the four preliminary encounters is one of Thunderspire's rare non-combat moments. Shortly after beginning to explore the Proving Grounds, the players come across the ghosts of a group of past adventurers who failed Baphomet's test. The three ghosts are essentially good guys, and are willing to share some information if they feel the PCs are equally motivated by unselfish goals.
The ghosts aren't exactly wacky Ghostbusters-esque spirits. They're ever so slightly more gritty. Each of the trio bear the marks of their death - one's head is crushed by the dragon's jaws, one's features are deformed by the dragon's breath, and the third has his torso rended by giant draconic claws. Throughout the encounter no one says the word "dragon" or reveals the nature of the Guardian, but the clues are there for players to begin harbouring some suspicions. It's great for players to see three competent heroes who have already failed at the task the PCs are attempting, and it builds the reputation of the climactic encounter well before its nature is even revealed.
As-written, interacting with the ghosts involves a skill challenge, with the ghosts interrogating the players and the players attempting to show their good intentions. I've never been a fan of the skill challenge mechanic and it's as clumsy as ever here. Rolling on Diplomacy, Bluff, and Insight seems appropriate, but, realising that that would leave one player doing all the talking, the designers have added extra skills. Players must roll Athletics to flex and pose for the pleasure of the martial ghost; Arcana to please the magical ghost with random trivia; but apparently not Religion for the paladin, presumably because Divine characters tend to come with Diplomacy as a class skill and will already have enough to do.
Personally I threw the mechanical element out the door and roleplayed it but I guess that's dependant on how entertaining you find skill challenges.
Here's what the ghosts reveal: to pass the Proving Grounds and progress to the inner sanctum, players must open the gate at the south-eastern end of the test. Doing so requires finding four holy artifacts of Baphomet and laying them simultaneously on the four holy circles scattered around the central complex. When the items are in position, the gate will begin opening and the Guardian will emerge to administer the final test.
One of the items is the Book of Wrath Unveiled which players have already liberated from the gnolls; the other three items are in the three rooms adjacent to the central arena. Each room contains a test that players must pass to secure the relevant artifact.
So this is the Proving Grounds - a slow and ominous exploration of the central arena, a conversation with ghosts, and then three gimmick-focused tests leading to a climactic brawl. This is a near-perfect example of how to build a satisfying traditional D&D dungeon, and how to manipulate pacing to have players genuinely excited about finding out what happens next. They'll be straining at the bit to try themselves against the Guardian, and thankfully Thunderspire - finally - doesn't disappoint.
 Actually, according to the printed text, the ghosts do reveal the Guardian is a dragon, if players get enough successes in the skill challenge. But it's such a vastly better set-piece if they don't that I'm exercising a kind of willful blindness. It's obvious enough from the injuries that there's a dragon involved; spelling it out in words just seems so crass.