Followers That Can Die: This was pre-Dawnguard also, but no follower can actually die unless you yourself kill them. I watched Bran the Dawnguard husky sit there and get giant icicles impaled in him at a sliver of health without problems, but when I accedentally bashed him with my c-bow it killed him.
Attacking Allied Beasts: My expirience was with Bran, where he recieved too much damage from me so he started to attack me and Serana. Bran has the inability to forgive you when you sheathe your weapon, so I used a calm spell and then told him to go home. Later upon arrival at Fort dawnguard, I picked up Bran from his pen and he was a faithful companion once again. Useful for the cretures that can die but wont respawn.
(This might work with trolls, if not oh well buy another one)
Crossbow Double Bolt Bash: My crossbow has been enchanted with a fire damage spell so it lights enemies on fire when they get bolted. However, when you bash with the c-bow they take minimal damage and no fire damage occurs. I discovered that if you repeatedly press the bash button while reloading the bolt, (kinda annoying for me because I have the zoom and slow time perks activated) you should immedietaly bash after reloading, but this time the bash does the exact damage of a bolt fireing along with the enchantment of the crossbow, it even makes the sound of a bolt hitting them instead of the bash sound.
So shoot a guy, he lights on fire, immedietaly after reload, bash, lights on fire again, shoot another bolt, lights on fire once more.
3 bolts for the price of 2, I call it the Double Bolt Bash.
Fast Werewolf perk progress: As soon as you can go towards the right side of the perk tree. Get "Animal Vigour" then "Gorging" and finally "Savage Feeding". With savage feeding you can feed on almost anything
Jesus Horse: Not necessarily Dawnguard specific, but if your on your horse who is swimming in water, simply dismount and remount the horse. When your on again you will be able to move as if you were on land, gallop and all.
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Dirty/Burnt face bug fix: The dirty/burnt face bug for vamps can be avoided by removing all eyeliner, eyeshadow, forehead and cheek color before going vamp or cure yourself then see the face sculptor and go vamp again...i found no cure for the dam lip line or sickly look.
(Thanks to Haggar)
Moth preist remains hostle to Serana during rescue: Not sure if its been fixed yet but bringing serana to save the moth priest causes bugs where the priest will remain hostile, even if you manage to finish the quest...tell serana to part ways before entering the cave he's trapped in and the quest should finish fine.
(Thanks to Haggar)
Serana Gimps VL drain life: If you plan on turning into a vampire lord, don't take serana with you as a follower. She gimps the vampire lords blood magic(drain life) spell and makes it significantly weaker.
(Thanks to Th3Rush22)
Rings of Blood Magic exploit: The ring of the beast and the ring of the erudite work in human form. The ring of the beasts unarmed damage bonus also works in human form in the giving the claws abilities. The ring of the erudites magika regen ability will not work during the day without a "base" regen amount such as from a robe. The ring I believe gives a 75% increase. You can also repeatedly equip the rings on yourself to recharge your health and magika if they fall below 100. This is useful when training your magic and armor skills.
(Thanks to johner210)
Werewolf feeding: Try not kill enemies in a pile, you will only get the feed option once.
(Thanks to Smas)
Bow and Arrow Bash: Double bolt bash applies to regular bows too, by the way. You just have to do it immediately after firing (or immediately after reloading if using an xbow).
(Thanks to Zexis0710)
I'll edit this post when I remember more, or discover new ones. Or when you guys add some.