But what is filler? When people say filler what they really mean was that they didn't like that part. If it had some meaning like having something to do with advancing the plot, then the whole game is filler.
As the word says, "filler" is what puts you to fill something. (No, really? Thank you, Captain Obvious!) I'd say it's the yeast used in a cake. It makes content expand, inflate, but adds no real flavor to what is there. Whatever you're doing in a game that does not require a significant level of reasoning is filler. Walking from one side of town to another, fighting random enemies that do not require any level of strategy, etc. If what you are doing isn't adding anything, if you take this to the "content" and the end result would be practically the same, it was filler.
Now one thing I find interesting is that, unlike what many here think, I don't think that everything that is beyond the central theme of the game is really filler - and AOD seems to be a game that will finnaly prove it. As the testers say, the game is very "raw". Something that doesn't have a direct relationship with what is happening in the story simply has no place within the game world. Everything must have a reason, all the quests offered make sense within the context of what is presented - which is not a defect in any way (just to be clear), but it ends up showing something very important about the games.
It may seem blasphemous to say this, but sometimes really is that "random" content makes the game whole. Just like a cake without yeast would not be the same thing, an RPG without that any other content beyond the relevant quests would also lose something. For many, the fun part of an RPG is exploring the city, find things, people, objects, weapons, challenges... Anything, even if it don't make much sense and has no direct relationship with the story. Even at the cost of loss of verisimilitude.
The "crime" that 90% of RPGs make is putting too much shit that has no logic within the story, making the experience too broad. In most RPGs where you would take 30-40 hours to complete, perhaps only 12 of these have any significant content. The problem is not the existence of filler "in itself", but the fact that it ends up being responsible for most of your experience with the game. If instead, the developers would merely present a game that has about 20-30% and not 70-80% filler content, the impression would be different.