For those unaware of it, Descent: Journeys in the Dark is a boardgame by Fantasy Flight Games in which the players take on the role of heroes descending into a dungeon. One player is the Dungeon Master... I'm sorry... Overlord, who controls the monsters. The other players are the heroes.
The rules are here: http://www.fantasyflightgames.com/PDF/descentrules.pdf
The game is one of FFG's "Big Box" games (along with the World of Warcraft board game) and contains a bunch of unpainted miniatures (heroes and monsters), cards for items, skills & runes ("spells"), and dungeon tiles.
Main similarities with D&D
* The aim of the game is to kill monsters, take their stuff, and get more powerful.
* Players go on quests - predefined adventures where the Overlord reads out boxed text, and places monsters according to the adventure text.
* The Overlord may design his own scenarios (there is an adventure editor online for this).
* PCs may return to town and sell and buy items.
Main differences with D&D
* The aim of the Overlord is to kill the characters.
* The Overlord is constrained in what he can do; a deck of Overlord cards allows him to spawn extra monsters, trigger traps, and have monsters act more effectively. However, he can't play every card he draws and much be careful about play.
* There is no interaction with NPCs/monsters save killing them.
* There are no guidelines for balancing self-designed scenarios.
In comparison to a game of D&D, Descent is a lot more constrained and less freeform. Gameplay for a scenario (of about 6 rooms?) tends to take about 4 hours, comparable to a game of D&D. However, it feels slower than D&D to me, and I find D&D a more rewarding experience.
In some ways, it would not be hard for D&D as it stands to be adapted into a game like Descent; take the Miniatures Handbook and the Random Dungeon rules, the Dungeon Tiles and the D&D Miniatures and you could probably cobble something together. In fact, I do believe that you could do a very good job of making what we already have into a good D&D boardgame (better than the Random Dungeon rules), which would add value to what is already there without removing the core D&D experience.
I do not think, however, that Descent will ever be as popular as D&D, nor should it be the model for a future main edition of the game. Quite simply, they play very differently. (I tend to prefer a different sort of boardgame when I play such games - Tigris and Euphrates, Caylus, Carcassonne or Settlers of Catan tend to be more my sort of boardgame).
Two of the other big contenders for "the fantasy RPG boardgame" are also made by FFG (World of Warcraft and Runebound), though I feel that are also a bit too clunky and involved to appeal to the mass market, although Runebound does the job better. (The best RPG-like game, IMO, is Arkham Horror, though it isn't without its problems).
I've played a lot of boardgames over the past few years; my friends and I have a good collection (60+?) There's a different attraction to boardgames than RPGs, and it tends to the more strategical and tactical type of player; they also do not involve the creative faculties as D&D does. If you try to make friends with the Ogre in Descent, the Overlord is just going to hit you with its club - there isn't going to be another option.
Cheers!

The rules are here: http://www.fantasyflightgames.com/PDF/descentrules.pdf
The game is one of FFG's "Big Box" games (along with the World of Warcraft board game) and contains a bunch of unpainted miniatures (heroes and monsters), cards for items, skills & runes ("spells"), and dungeon tiles.
Main similarities with D&D
* The aim of the game is to kill monsters, take their stuff, and get more powerful.
* Players go on quests - predefined adventures where the Overlord reads out boxed text, and places monsters according to the adventure text.
* The Overlord may design his own scenarios (there is an adventure editor online for this).
* PCs may return to town and sell and buy items.
Main differences with D&D
* The aim of the Overlord is to kill the characters.
* The Overlord is constrained in what he can do; a deck of Overlord cards allows him to spawn extra monsters, trigger traps, and have monsters act more effectively. However, he can't play every card he draws and much be careful about play.
* There is no interaction with NPCs/monsters save killing them.
* There are no guidelines for balancing self-designed scenarios.
In comparison to a game of D&D, Descent is a lot more constrained and less freeform. Gameplay for a scenario (of about 6 rooms?) tends to take about 4 hours, comparable to a game of D&D. However, it feels slower than D&D to me, and I find D&D a more rewarding experience.
In some ways, it would not be hard for D&D as it stands to be adapted into a game like Descent; take the Miniatures Handbook and the Random Dungeon rules, the Dungeon Tiles and the D&D Miniatures and you could probably cobble something together. In fact, I do believe that you could do a very good job of making what we already have into a good D&D boardgame (better than the Random Dungeon rules), which would add value to what is already there without removing the core D&D experience.
I do not think, however, that Descent will ever be as popular as D&D, nor should it be the model for a future main edition of the game. Quite simply, they play very differently. (I tend to prefer a different sort of boardgame when I play such games - Tigris and Euphrates, Caylus, Carcassonne or Settlers of Catan tend to be more my sort of boardgame).
Two of the other big contenders for "the fantasy RPG boardgame" are also made by FFG (World of Warcraft and Runebound), though I feel that are also a bit too clunky and involved to appeal to the mass market, although Runebound does the job better. (The best RPG-like game, IMO, is Arkham Horror, though it isn't without its problems).
I've played a lot of boardgames over the past few years; my friends and I have a good collection (60+?) There's a different attraction to boardgames than RPGs, and it tends to the more strategical and tactical type of player; they also do not involve the creative faculties as D&D does. If you try to make friends with the Ogre in Descent, the Overlord is just going to hit you with its club - there isn't going to be another option.
Cheers!