Too many loose ends in each storyline (e.g. upon returning to the present from 1795).
[spoiler]Barnabas suddenly deciding he "loves" Angelique after all she did to him. (Sorry, Terry308.)
Vicki getting killed by Jeb Hawkes. [/spoiler]
I agree with those, most especially the first one in the spoiler tags. I haven't actually seen that storyline, but the knowledge of it drives me absolutely crazy.
Also for me:
-The lack of use of the character of Willie, by far one of the best characters on the show
-The Adam and Nicholas Bair storylines.
-Not enough screen time for Barnabas and Josette as a couple in 1795
-The way it became less about the present day Collins family (Elzabeth, Roger, etc) and more about all the supernatural characters after 1795.
-The fact that good potential couples never got more development
-Peter Bradford/Jeff Clark
My own spoilerized pick:
[spoiler]Nicholas erasing Maggie's memory of her abduction by Barnabas. The writers really took the easy way out and that was completely disappointing for me. That would've been a better storyline to explore than the continuation of the Adam and Eve and Nicholas stuff.[/spoiler]