I'm thinking of purchasing this as my next Fantasy Flight Game.

How does it compare to the old "Hero Quest" game? The description seems similar, is the overall concept the same?

hmm...i would say, descent is hero quest 4.0

They are similar in feel and theme. If you liked HeroQuest there's a very good chance you'll like Descent as well. The big difference between them, IMHO, is that HeroQuest was balanced with the intention that the heroes would win the scenario. Descent is meant to be a fair fight between OL and Heroes. If the heroes approach Descent with the idea of killing everything that moves and picking up every single piece of treasure, they WILL take a beating for it.

I'd also like to mention that the "overlord"/DM is much more engaged in Descent. I heroquest, the Overlord was fairly passive: you reveal rooms, move monsters, make attacks.

In Desent the Overlord is more actively trying to thwart the hero players, by activly choosing what monsters to spawn, what types of attacks to use, and playing special overlord cards to defeat the players. (If you get expanions there are even more ways for the Overlord to customize their deck and options)

May also be worth noting that Descent quests are designed as one-off games, where the heroes gain substantial power during a quest but get reset for the next game, unlike HeroQuest where the heroes retain gold and equipment from one quest to the next. There's an expansion for Descent that turns it into a campaign (Road to Legend), about which I can't say much.

Also, unlike in HeroQuest where you could generally take as long as you want provided there aren't any monsters on the board right now, Descent puts the heroes under constant time pressure--the overlord can prepare more traps and monsters and such the longer you take. So the dungeon feels a little bit less like exploration and more like a race.

Note that I haven't played HeroQuest in a very long time.