From the description, I couldn't really tell how different this is from the current edition. It says that its deadlier and includes a Torchlight variant.

The only thing I see different is the cards for the monsters. The combat system looks the same (still requires a monster player, has a "combat deck").

So, what will be different about this version? Is this just for people who don't own a copy and can't afford it on the aftermarket?

From http://www.fantasyflightgames.com/edge_minisite_sec.asp?eidm=263&esem=1#different

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What’s Different in DungeonQuest Revised Edition?

We’ve listened to the feedback since the release of DungeonQuest Third Edition and while working closely with designer Jakob Bonds, have implemented numerous important changes in DungeonQuest Revised Edition. Some of these changes are detailed below:

Combat has been simplified: Combat has been redesigned to feature a streamlined combat system that harkens back to the classic editions of the game. DungeonQuest Revised Edition features a card version of the combat chart found in a previous edition. The new combat system delivers the same tension that DungeonQuest players love, but allows combat to be played out much more quickly. For more detailed information on combat, please see the “Combating the Dungeon’s Monsters” section above.

Search once per room: In DungeonQuest Revised Edition, rooms in the dungeon can now be searched only once per game. This encourages brave dungeon-delvers to venture deeper into Dragonfire dungeon, and is easy to track by placing a search token on the board.

Move between towers: When a hero enters a tower room, he can now optionally move to any other tower room and continue his turn. This change allows you to shadow other players who have created successful paths through the dungeon, adding a new layer of strategy. This also prevents you from being stuck in the opening turns of the game.

Escape using Agility: This streamlined escape mechanic allows players to test Agility in order to successfully escape from a combat.

New rulebook format: DungeonQuest Revised Edition takes advantage of our new rulebook format. The rules are split into two separate, streamlined documents–a Learn-to-Play guide, and a Rules Reference book. This means that you’ll be able to begin exploring the dungeon as soon as you open the box with the help of the handy Learn-to-Play guide. The Rules Reference guide will answer any questions that you may have as you play the game.

A deadlier dungeon: Dragonfire Dungeon is deadlier than ever before, adding new challenges for seasoned dungeon explorers! Surviving the dungeon is a heroic feat, and only the bravest of the brave will make it out alive.

Torchilight Variant: When using the Torchlight Variant, when a player moves into an unexplored room, his torch casts enough light to make out what lies ahead. As a result, he also draws and places adjacent rooms that he has line-of-sight to. This variant adds an element of strategy to the game, as well as creates a much fuller and more complex dungeon throughout the game.

As far as physical component differences, it sounds like the only changes are a new card-based combat system to replace the old and adding search tokens that are placed/removed, rather than having the search icon printed on the tiles.

And there's some mention of new, deadlier traps, so some of the challenges may be new and/or tweaked.

The rest all sound like rules changes that don't really require new components. As such, I suspect it would be easy enough to just download the revised rulebook and play with your existing edition. You'd have to continue using your old combat system (or combat variant of choice - there were many, IIRC) and you'd either need to find your own search tokens or ignore that rule and go back the old system for searching.

I'm not sure how I feel about the Torchlight variant. It makes sense that your hero can see what he's getting into before he steps through the door, but I worry that it might become too easy to avoid those hilarious "I got trapped in a rotating room that goes nowhere" scenarios