Ripped from the other Unfinished Collections? thread, I'll head in the direction I thought it was going with I first saw the title.
Are there any titles you began to collect that after you had started you didn't care for or thought better of and just couldn't bring yourself to finish up? I have a few that are still taking up space on my shelf. I really should use them for frisbees or something:
Wedding Peach (Volume 1) - Had a hard time just getting through the first volume. Seemed to repeat the same formula episode after episode.
Steam Detectives (Volumes 1 and 2) - I gave this title two volumes to sell me but I never really could get interested in the characters. Perhaps I need to revisit this one.
Wandaba Style (Volume 1) - Only took one volume of this series to turn me away from the uninteresting fanservice fest that it kept dishing out.