D2:ED didn't use DirectX 10. I don't know if the updates in DKS take advantage of any DirectX 10 features, but I don't recall seeing that mentioned anywhere.
What was the end of the demo? Assuming you get access to the full Broken Valley, the fjords is another map about that size, and there is an island about the size of the starting section of Broken Valley. I don't have DKS yet, but before the Hall of Echoes there are limited places in Aleroth you can go; afterwards (the FoV section of the game) the city opens up. IIRC the official playtime estimate for D2:ED was 40 hours (I explored everywhere, etc, and took 75 hours), while FoV was 20 hours.
Some people have said the game wasn't replayable because the main plot quests don't branch (many of the side quests do, and there are multiple ways of solving quests), while others have played the game dozens of times. Unless you follow a walkthrough, and make all the same choices the second time you play, you are most likely going to discover things on a second playthrough that you missed the first.
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