I agree, incomplete stories are annoying, but knowing the reason makes it a little easier to accept them - and it makes me a little sad too. I have a friend who is a new mangaka for the shounen/seinen genre. He has several works published, but hasn't had a full series yet. Every one-shot is a trial run for a series unless the editor is specifically looking for a one-shot for an anthology or to fill in gaps in a magazine. When the one-shot is successful, gets high ratings in surveys or high sales, they continue it as a series. The way we read manga here, where the entire story is composed and compiled together as one continuous story, isn't how they are commonly published and distributed in Japan. Full stories or "Takouban" are published when a series does well and the publisher wants to make more sales from the story's popularity. Most popular stories are spread out with one chapter being sold together with chapters of other stories from other mangaka in a magazine form. Since this site is compiled by author, we'll see one shots by themselves and the same one-shot in a compilation. So, basically, one-shots that feel incomplete are failed starts for their own series. That, to me, is the sad part. The mangaka worked hard to plan out a whole story around these characters and it never went anywhere because the sales or surveys weren't good enough. My friend keeps all his "failed ideas" in a file cabinet all their own and it is full to the brim. Being a mangaka is a really hard job that can make you feel terribly insecure. He is lucky that he is very self-confident and optimistic. I don't think I could survive that kind of criticism. I dislike incomplete feeling one-shots as much as you do, but I think it is for a different reason. LOL
These were one-shots with story lines that weren't meant to be one-shots, they just ended up being rushed and feeling shallow. Can't get in to the characters or feel anything for them. When one-shots are done well, they're satisfying - just enough sex, and a story short enough to fit into one chapter. These could have all been fleshed out and made into longer pieces, to get a better feel for the mood and the characters.