The most obvious cue? The perspective, which has gone from the zoomed out view the series is most associated with, to a more traditional third-person action perspective right over the shoulder. The classic view is still available with just a button press, but most of the demo I saw zooms in tight on the lead character.
The message is pretty clear: This is the new face of Dungeon Siege, and it's doing its darndest to win me over.
There are other modernizations as well. This being an Obsidian game, the studio is bringing its talents in creating branching dialog for this new iteration. Though co-op's not a new feature to the series, it's been streamlined since DS2, with a friend able to jump in and control your partner at any time without interrupting the flow of action.
It would be understandable at this point for long time franchise fans to worry that they were going to be left behind, but the core tropes are still very much in place. Fighting off hordes of enemies while collecting gold and loot is still very much central to the experience. And Obsidian promises that while the team hopes to keep the writing quality of its previous titles, there won't be enough of it to get in the way of the aforementioned loot gathering and monster slaying.
It's early yet, and Obsidian still has an awful fine line to walk, but at this early stage it seems that the studio definitely has a shot at reaching this franchise newbie.