So you're not going to finish the Tremulous one? But we're left hanging after Chapter 3!

It's okay if you want to do something more interesting, doesn't matter. Maybe if you're going to not write novels, you should aim to do short stories that do have a begging middle, and end -- all self contained. Short stories used to be my favourite kind of writing, but I always had trouble sitting down and writing a lot of stories. I tended to get distracted and work on other stuff.
Also, a criticism I received in my literature writing days from my English tutor was that my characters could use more depth and personality. I think that is good advice for you as well. In general, some things to consider would be:
- Using a stereotype is an easy and fast way to establish a large mental image of a character in the minds of the reader. Perhaps you describe the person as meek with downcast eyes but with the habit of frequently looking at people darkly as if with dislike. Just from that, similar characters would pop into the minds of readers and flesh out the personality. Perhaps the housewife with a calm and quiet demeanor and some overtones of being abused. This kind of thing just springs to life.
- Some hardcore English people say that characters aren't worth much unless they have some element of character development as the story progresses: They learn some moral, change in some way. This is also a good way to weave in messages to the reader.
- It is worth adding extra thoughts and actions that do nothing but expand the character depth. You could talk about how the character is addicted to smoking but has a severe dislike to smokers and carries his burden as self-punishment for some misdeed he did, which perhaps is revealed not all at once to create suspense. Or you could give the characters some kind of common phrase that they like to use, frequently describe some gesture or their tone of voice. Perhaps you could have a character see something and trigger a memory sequence that shows what kind of person they are, etc. Details count. The idea is to have the reader connect with the characters on a personal level so you really get into the story... and feel it so much more when they are hurt or die.
Anyway, that would be nice. I also recommend the works of Asimov. He's my favourite Sci-fi author and he is really really good at it. His stories tend to have a lot of threads that weave into each other delicately, and his Robot series in particular has amazing characters that you really connect to. His endings also tend to be really unpredictable, but after you read them, you see that he's laid out all the clues, you just couldn't put them together.