For one thing, because it's 4 times bigger than doom :P

Sure you have to like the medieval theme...

One part is character customizaton, and the second part is a more robust and customizable combat mechanic.

Characters in Descent each have their own unique ability, and then have up to 3 skills. This combination of 4 intertwined abilities can give you some very interesting combinations, and the option to learn new skills just adds to this.

However, even more attractive is the combat customization.

The dice in Descent have "surges" on some of the faces, which are used to power up a weapon's special abilities at variable rates. One weapon might convert two surges into one range, while another weapon might convert surges into damage at a 1-for-1 ratio, while another weapon might give you multiple options.

Furthermore, there are the "Power Dice" which give you various faces, some of which roll surges, but others of which give you "power enhancements", which give you the choice of adding +1 to either range or damage.

All of these factors combine to give you some very interesting choices.

Do you choose the weapon which gives you lots of choices to spend your surges? Do you choose the weapon which gives you limited choices but lots of damage? Do you choose the skillset which gives you lots of special abilities from a positional viewpoint, controlling the monster's position?

My game last week had the heroes moving away from a straight-damage perspective, assigning various roles to various heroes, as they became the point-man, the crowd-control expert, the "set-'em-up" guy, the "move-'em-around" guy, and so forth.

Now, it isn't just rolling the dice and seeing the result. It is rolling the dice, seeing the result, and deciding how to spend that result to get the result you'd like. Because even after the roll, you might channel surges one way, or another, or even buy more power dice to try and push your attack over the edge and go for the kill.

Compared to that, DOOM's "Roll the dice and see what happens" style seems rather lackluster when you go back and play it. It just seems less engaging, and less interactive.

OUT OF AMMO!

If you don't like the theme, I'd say try before you buy.
I personally prefer the simple streamlined rules of Doom over the added chrome in Descent.
While the adventurers will be upgraded several times during the game, I don't see this adds to the general tactical decisions of the game. It does however give the game a slight fantasy RPG feel.
The scaling of the game is also quite different. To my experience the heroes will have a better chance of surving, the further into the dungeon they manage to go.
The flow of the game is comparable to Doom, if you play with experienced players. If playing with new players, there will be more downtime in Descent, as the players try to find the best way of distributing the treasures they find.

Well, first of all there are plenty of people (like me) who like Doom over Descent, so you may be over-generalizing a bit.

Doom is a rich, thematic survival game that forces players to keep moving and uses a brilliant combat system.

Descent is a rich, thematic dungeon crawl that allows players to explore and uses a modified combat system to Doom.

Take your pick, they are both good in different ways.

well lets see...
(1) you get to control more than just marines, you get many different heroes, all with different stats.
(2) you get a better selection of skills, 3 decks to be exact!
(3) better scenarioes, instead of "exit this place" you get much better variety, whether you kill a terrable dragon, or rescue a princess, or even save the world!
(4) 2 types of monsters, normal, and their diabolical masters!
(5) better items, in doom you get like, 10 weapons tops, in descent you get tons of them.
(6) NO AMMO!
(7) dooms frag points were annoying to keep track of, in descent you got gold to buy things.
(8) And finnally, IT IS BALANCED BETTER! the doom game, marines had to run, barely able to kill, and the invader could play cards freely, in descent you can actually take your time, and the overlord has to pay to get cards.

those are my thoughts, and i hope i helped.

Though we prefer the theme of Descent, Doom hits the table more often and is more fun. Go figure. I think it's the streamlined mechanics and shorter game lengths. In Doom you're choosing between all your different weapons every turn and agonising over ammo. In Descent you generally have one weapon/combo equipped and just keep using that till you upgrade to the next weapon/combo.

I think Justin was talking about Kill Points rather than Frags? Cos anyone who struggles to count up to 6 shouldn't be playing either game!

I wish I liked Descent more, but Doom is the better game, it just needs more support and missions . . .

My group liked Doom more but creating missions in Doom is time consuming.

The reason why we've only been playing Descent and not Doom lately?

Descent: Road To Legend. We actually tried to create our own campaign rules for Doom long before Road To Legend was even announced, but the endeavor put way too much work on my hands (Overlord) to create the world map and a handful of missions.

Road To Legend is amazing and I don't see us ever playing vanilla Doom or Descent again.