5. The Vampire Lord is absolutely useless and a gimic
The Vampire Lord is a worthless gimic. The fact that you are forced into 3rd person mode really kills the immersion and the perk trees for it are bad. Not to mention the bug issues that occur with said feature such as not being able to walk into a door.
4. Illusion of Choice between Dawnguard and Harkon's crew
Unlike Shivering Isles where you got clear choices between the two houses you don't really have that with Dawnguard. Serana sticks with you regardless of what faction you choose, Crossbows and Vampire Lord is really the only main choice between the two factions. This I have a problem with because I would like clear choices between the two such as specific weapons/armor for choosing a particular side. I would want my choices to matter unfortunately that doesn't happen with Dawnguard.
3. Bugs, Bugs, and more Bugs.
I swear did Beth kill the Orkin dude because Dawnguard is litered with Bugs and don't get me started on what Beth's doing with the PS3 version of Dawnguard, completely unacceptable although that's a different discusion for a different time. There's also a huge bug that's involved with number 1 on my list

1A. Radiant Vampire Attacks in walled cities.
This one is absolutely terrible, I don't know who's idea it was to add this feature but this is a terrible decision. It's great in theory but there's one small problem, NPC's in Skyrim are weak. Obviously that happened because of Oblivion's overuse of Level Scaling and it doesn't help that said NPC's don't wear any [censored] armor. Merchants are the worst especially adrienne in Whiterun. There's two major problems that occur because of that, merchants don't respawn and they have finite gold. Unless your with the College or The Thieves Guild you will probably run out of merchants or be down a good amount of merchants. This is probably the easiest problem to solve, just make all the merchants essential to everyone but the player.
There's also one last big issue I have with Radiant Vampire Attacks in walled cities. It basically forces you to do Dawnguard, we've never have an Elder Scrolls game that forced you to do a DLC if you installed it. This would be like Sheogorath giving you visions that you must save Shivering Isles from the Greymarch before you even step through his portal and it occurs every 24 hours. That is just terrible, I would like to have Crossbows without deciding this little Vampire War or breaking my main quest which I will get too with Point 1.
1. The Dragon Scroll
This one is beyond the pale bad. This is the item that we need to get in the main quest and we also have to get it for Dawnguard. If you get the item before you get to that point in the main quest it breaks the game badly. I don't understand why they couldn't use a different item. This is basically equivilent to getting the Great Welkynd Stone in Oblivion but I have to give it to Haskill so that he can use the energy from the stone to power up the Staff of Sheogorath. The problem is that also breaks the main quest for Oblivion if I get the Stone before I'm suppose too.
In Summary the Random Vampire Attacks are the biggest issue, a complete lack of common sense on Beth's part I mean Merchants have Finite money but on a fundamental level the Dragon Scroll, an item that you have to get in the main quest that is also required for Dawnguard, that really bothers me. Hopefully that's not a trend for the upcoming DLC's, I mean what's next, The Horn of Jurgen because a midden needs the horn to get rid of a rock.