I think the issue is that Obsidian has tried to make Dungeon Siege 3 something that it's not. I get that they are big on story etc, however when it comes down to it Dungeon Siege was simply a Diablo clone, nothing wrong with that.
What myself and I'm sure many others enjoyed about the original was the online / local co-op of up to 8 players, massive amounts of loot, huge modding community, no level cap (it was 150 in the original but impossible to get due to the crazy amounts of xp required for levels 120+), etc.
The arguments many have provided supporting the decision of this style of co-op would be valid if it was a completely new IP. However at the end of the day this is Dungeon Siege, a series that already has an existing fan base, largely due to many features that are not present in Dungeon Siege 3. That's the issue when it comes to this crazy choice of co-op Obsidian has chosen. Although I do admit that even if this wasn't Dungeon Siege I would see this form of co-op as being pretty stupid.
As someone mentioned earlier, Resident Evil 5 was a game that was story centric game that allowed players to keep their own characters and progress... why couldn't of it been handled like that? Yes that does mean the ability for a level cap character to run through the game with a level 1 character, but who honestly cares? If you want to go through the game with the intended difficulty, no one is going to stop you from doing so.
Honestly I don't think they have respected the original fans of the series. DS3 is missing a large chunk of the features that made many of us fall in love with Dungeon Siege all those years ago. Unfortunately this seems to be one of those 'sequels by name only' types of games having almost nothing to do with the original. This game should have received a different title (from what I gather many of the people supporting the decisions Obsidian have made never actually played the original Dungeon Siege) if it's not going to stay true to it's predecessors.
At the end of the day, we are going to have to learn to accept the game for what it is. I have no doubt that I will find the game fun and enjoyable, however it won't be anywhere near the game I had hoped it to be. It's just unfortunate that I and many others associate the title of this game with something completely different.