Some of the above annoy the hell out of me: Over-powerful characters, complete lack of research (in anything - I mean, come ON, we have the internet, for crying out loud), bad spelling, bad grammar (how many grammar tutorials/websites are out there?), entire conversations in another language (with no translation), songfic where half the fic is simply the lyrics, and starting a story and not finishing it, and disappearing.
All of those annoy me to no end.
However, some things I'd love to address myself:
Sex involving a minor - first off, the word "minor" is very subjective. Minor according to whom? The US? Canada? Where in the US? Age of consent for sex is different from one state to the next. I was 15 when I had sex the first time, with a man I was very much in love with. I was also mature enough to handle it. That was 21 years ago. Minor in, oh, feudal Japan? In that case "minor" would be...oh.... 10. They married at 13. (And sometimes that was considered old). And a month before a birthday? Is that ONE month REALLY going to make that much difference in their maturity? Being able to handle sex isn't about the number. It's about the maturity. Please.
Happy Endings - Now, I have no problem with happy endings. In fact, I rarely read stuff if it's GOING to end badly, and I know that, though if I know it, then I'm less likely to have a problem with it. No, life isn't always happy. But that's why I read. I don't read for reality. If I want reality, I can turn on CNN. If I want reality, I'm not going to read about a completely different world full of ninja who use Chakra to do things that are otherwise impossible. I'm not going to read about a demon who has a good heart, dog ears and loves a human who isn't going to be born for another 500 years. Having a problem with a happy ending just because its happy pisses me off to no end. Killing a character or making them break up and be unhappy for no other reason than happy endings don't exist is just ridiculous. That kind of writing can cause me to not read anything of that authors work ever again.
Oh and on verb tenses, Ezriee, you are right on the mark. Present tense also annoys me to no end. It is extremely difficult to write well in present tense. So, honestly, it's best to avoid it altogether. I can understand why someone might want to - the idea of trying to help them experience something first hand, but if the rest of the story is written well, the reader will feel like they're firmly involved, anyway.