Part of the problem here is that Bethesda just sets the bar too high for itself. I had played about 500 hours of Skyrim before getting Dawnguard, so that game alone was really worth more like $300 to me, yet I saw a lot of people complaining about it in these forums for weeks after launch. People are asking for depth out of all proportion to any other RPG franchise out there, yet they expect it to come at the same ticket price as every other game.
Now, compare Dawnguard with Skyrim and you might feel that it's too short ... but that's because you're hoping that for a third of the price of Skyrim you will be getting a third of the content of Skyrim, which is just insane. They gave us too much first time! Compare Dawnguard with any other DLC out there and it's more than adequate.
Dawnguard is not mind-blowing, and there are things that annoyed me about it: in particular, the heavy use of the Radiant Quest system to pad out its running length. But I actually think that we view some of the other expansions with rose-coloured spectacles as well: The Shivering Isles has been the best, but there's a lot to dislike about that one as well. For me, Dawnguard is probably better than Kot9, Bloodmoon and Tribunal, and the key thing for me is that everything that they did put in it as new content is pretty cool.
Whether it's worth the money varies between consumers, depending on how much they have and how much they enjoy Dawnguard. All I can say is that it was worth $20 of my money, and I'd happily have paid more.
Just like I've mentioned before. You do realize that the money Bethesda receives for each copy of a $20 Dawnguard is almost equal to that of a $60 full game right?
People still don't realize how lucrative these DLCs are. Yes the cost of producing video games has increased(mostly due to those aggressive advertisemants though, just look around and you'll find a lot smaller game developers who don't do too much advertisemants able to go on with a much smaller audience), but the benefits from DLCs have more than negated that.
DLCs are so lucrative that even if only 1/2 of the players who bought the original game buy the DLC, the publisher will still get more(much more) money from the DLC than from the original game. Not to mention that the cost of making a DLCs is much lower than that of a full game.