One major one is AI pathing, now I know pathing was an issue prior to Dawnguard, but now it seems alot worse. I notice if I'm suppose to follow an NPC, sometimes he/she will stop turn around and look at me/wait for me to get close to continue moving. However this is exceptional bad now becuase sometimes he/she will just sit there looking at me, sometimes upwards of several minutes till finally they will move. Sometimes it gets so bad I have to reload a save and pray it doesn't happen again. There is another example but I will add it at the end of my post seeing as it contains minor spoilers and I would rather not ruin it for people reading this
Other bugs include:
Corpses of enemies occasionally following through the world, completely disappearing. Random followers disappearing when leaving/entering locations and being completely gone. NPC's constantly saying to me to stop using dangerous magic, even though most of the time I don't even have anything on to have them say it. A minor annoying glitch I found, this is only if you have subtitles enabled, both options. Whenever you have alot of subtitle text on the screen and you happen to load a saved game, sometimes the text stays on the screen for an unknown length of time. I've even had a rare occurance of the text when entering locations staying on the screen but it only happened once so far.
Will add more when I find them but this is it for now.
Read below for glitches pertaining to storyline or subquests and are considered spoilers:
First glitch to add is Barbas literally follows to close. He's always in my butt. Sometimes he "pushes" me and barks repeatedly to the point he's spazzing out. It's very annoying and should be looked at.