Well, yes, those two FO3 DLCs were awful. But let's not pretend that Honest Hearts and Lonesome Road were masterpieces either. I'd say each game only had two worthy DLC: The Pitt and Point Lookout for FO3, Dead Money and Old World Blues for NV. The rest could have been done without.
Only Honest Hearts came under some fire because it was kinda dull. You felt like you got your money's worth, but it was barely worth it and lacked a "wow" factor like the others.
Lonesome Road, yes, I'm aware of the scathing reviews, but it makes perfect sense for the DLC as it's not really something a reviewer (who regularly reviews very different titles) can pick up and understand what an impact the DLC has for people who followed New Vegas to the end. The FANS (people who followed New Vegas from beginning to end) love it, everyone else thought it was meh; makes all the sense in the world as someone who buys Lonesome Road without understanding the meaning of anything Ulysses says or any of the meaning behind events in the main game AND the story that stretched across the DLCs? They won't know what they're missing or what's going on and thus won't like the DLC, thus the low ratings on reviews, albeit you would have no clue reviews were low if you visited the New Vegas forums.
And even so, people refer to Zeta as an abomination. Zeta and Anchorage get far more criticism than HH.
So again, newness has nothing to do with it. Dawnguard is being insulted because it svcks, not because it's new.