The players may have defeated Krand and finished the Chamber of Eyes, but there's a question still remaining: where are the slaves?
The whole point of assaulting the Chamber and grappling with the Bloodreavers was to rescue the innocent surface-worlders who the hobgoblins had enslaved. It's annoying, then, that when the mini-dungeon is complete no rescuing has occurred.
It turns out the Bloodreavers have sold the slaves to a nearby faction of Duergar (evil dwarves). There are a couple of random Duergar included in the last battle to help set up this link, and if that's not enough for players to put two and two together, Krand (like any good villain) is carrying a note on his body to make sure even a half-ogre can tell where the plot will be going next.
That's great for moving the players on to another enemy-packed killfest, but from the players' perspective it's unsatisfying. They've just slaughtered their weight in hobgoblins, and at the end they're no closer to achieving their aims than when they started. The goalposts have moved.
The combat is good in 4th Edition - so good it can lead to backwards thinking. It can become tempting to use the plot as the way of leading players to the next fight. That's the reverse, of course, of using fights as a means of furthering the plot. It's a trap that Thunderspire falls into both here, and over the course of the next mini-dungeon.
Combat should be about something. It's more that just the triumph of the players over the monsters - it's also about the triumph of order over chaos, or community over selfishness, or life over death. At the end of each and every battle, players should feel that they've achieved something larger - that the world has changed as a result of their actions.
You can't move the goalposts. When your plot has a climax, and the players survive it, you have to give them something. Whether it's a horde of cheering villagers or the discovery of an ancient and powerful secret, the final defeat of a hated rival or just blowing up the dungeon after they leave, there has to be both closure and progress. It's not acceptable to just say, "Great, but can you do it again with dwarves?"
If you're running Thunderspire there are a couple of ways you can mitigate the damage here. One is to foreshadow that the slaves have been on-sold long before players get to the Chamber; their goal then becomes not to free the slaves but to discover their location, which is something they'll fully achieve. Another way is to emphasise that the slavers are about to mount a new expedition - this time to the Seven-Pillared Hall, or some other place that the players care about; by cleaning up the hobgoblins the players have saved named NPCs who matter to them from a horrible fate.
Whichever way you choose to go, though, you'd best make it good, because there's still a long slog ahead through the Horned Hold before we finally get to what passes for Thunderspire's "good bits". In the mean, I hope you like Duergar, because everything we just saw in the Chamber we're about do again. With dwarves.