Throughout the story, I was expecting two things to happen:
1) I was expecting Isran to be a psychotic [censored] who tears the dawnguard apart almost and then rushes into battle against Harkon, only to get himself and most of Dawnguard killed.
2) I was expecting Serana to betray me and try to kill me once I had all three scrolls and the bow.
Neither of these things happened, and that's a good thing, cos those two plot devices are so overdone in lately.
My only gripe is that, after making Serana human, I will not be able to marry her, which seems odd considering how close they seem to become throughout the storyline. Is there a chance of this being "fixed" at any point? I like the idea of my hero character convincing Serana to step away from the darkness and for them to fall in love and marry afterwards.
EDIT: Also, is there a level requirement for starting the Dawnguard questline? pretty sure I heard/read somewhere that it was level 14, but I'm not sure.

The indicators so far are if you have not completed Dragon Rising, completed Dragon Rising and completed the main quest, ie slaying Alduin. 