The guy that made Descent is the same that did Arkham (together with the original GW era designer) as well right? Kevin Wilson if I'm not mistaken. Not that it should make both games equally good for that matter.
Yes, Kevin Wilson was involved in AH, but was the sole creator for Descent. Two totally different games, though.
As I've understood, there are two versions of Descent out there. While you have regular Descent with the scenarios for the main game and with included expansions. And then Road To Legend where you're playing on a map instead and have encounter on different locations on the overview map. I would guess same rules regarding combat etc. Have you played RTL Petru5? I'm thinking of starting slow, just learning the basic first, and go on to the expansions chronologically.
The base game of Descent and all the expansions except RtL are campaigns unto themselves. So, for instance, Tomb of Ice has similarly themed dungeons that follow a story path to a conclusion. Your characters, however, 'tank up' each scenario within the campaign, but start back at square 1 when they begin the next scenario in the storyline. Also, each expansion introduces more rules, some of which are specific to that campaign. Like AH, the rules can be daunting at the get-go, but time and experience cures that.
RtL breaks the storylines into manageable chunks (as short as an hour) to alleviate the 3-4 hour sessions needed to play one scenario. Characters also can 'level up' using a campaign system.
My recommendation is to try the base game to see if you even like it. Then get RtL to make the game more fun, give it flow and make it time savvy.
I have got to get myself some Doom.. I've just heard some bad stuff about it, but that it is much better with the expansion.. I like hard games though. 
The expansion integrates some of the rules from Descent in regards to set up and Overlord card play. The figs are swaps for the base game's (e.g., one of Creature X is = one of Creature Y), but are beautiful in their demonic way. I like the game because of the feel of the game. The Marines are in deep doo doo from the get go and really have to balance unit cohesion, speed and luck in order to even make it to the final room and out the final door. It's one of those games (like Pandemic) where you know that you'll probably lose, so the fun is to see how long you can hold off failure. The occasional player victory really makes you feel like you've accomplished something big! Again, try the base game to see if you like it, then add in the expansion.
There's a guy that runs a giant Space Hulk game at Recruits using a large 3-D board. It's awesome!
