First off, your opinion to me is void because you don't seem to be a recurring Elder Scrolls fan since you would realize from previous DLC (cough cough Oblivion cough cough), that Dawnguard is well worth $20.
That makes absolutely no sense. I think what you're
trying to say is that 'Dawnguard is much better compared to Bethesda's previous Elder Scrolls DLC and if you were a fan you'd know that', which is not an argument.
Second, just think about how you spend your money on other things, a 2-3 hour movie costs almost half of DG, and yet for Dawnguard you are potentially getting around 20-30 Gameplay hours.....Now if you choose not to play both sides of the DLC, you can't complain because you should've done your research.
This is an argument, but a poor one. Fair enough if you spend your money based solely on time, but OP's argument seems to be that there wasn't much added in terms of new gameplay elements. I happen to agree.
I like Dawnguard! I hadn't picked up Skyrim in a while so it was an excuse to jump back in, and I'm having fun so far. But it does just seem to be more, rather than new.