I could write a dissertation about my personal feelings regarding fanfiction pairings, so I will try to keep this short. I read mostly Harry Potter fanfics because growing up Harry Potter was really important to me, but as I got older and the final books were released I became less impressed and less happy with how things turned out. It was really crappy what happened to Snape and I know and have finally accepted that maybe he just didn't want to live at the end/he was perhaps supposed to die, but I just wasn't happy leaving it that way. (off topic but as a twin myself I found JKR killing Fred to be impossibly cruel) So I prefer to read HP fanfics where things end a little more happily. Some people seem to think Snape should be paired with anyone but I really think Hermione is the best person to pair him with (which I could go on and on about but I'm going to just leave it at that). It really is my only exception to my dislike of paring older witches/wizards with younger ones. But this could also be due to the fact that I always did like Snape and was more than a bit of a Hermione growing up.
I will also read Snape/Lily but only if I'm looking for something a bit angsty. I used to prefer where things ended up happier between them but after seeing others opinions, I have to agree that Lily was probably a bit of a vain girl who strung poor little Severus along and used him for good grades and for his knowledge of the magical world. My opinion is you can't be related to Petunia Dursley and not be at least a little bit of a status whore yourself, James Potter being interested in you is a big deal if you had plans on "moving up" in the world.
Other than the occasional Hermione/Draco (and the SS/LE), I really do not like seeing either SS or HG paired with anyone else. I really don't have a preference for any other side parings or tropes, (Though I do admit I'm always a sucker for stupid Harry and Ron, Jerk Ron, and Bitchy Molly.) though like I said I don't like any one of Harry's generation paired with those of his parent's generation or older. I know Harry/Hermione and Hermione/Ron are popular pairings but I don't feel like she's suited to either of them, they were only friends due to circumstance and they needed her for homework help, they never did seem to have much in common to me.
A pet peeve of mine is when a story ends up too happily ever after, and we find out how many great grand kids everyone had and what their names and spouses were. I prefer when there is no childbirth or the story ends before that's ever an option, that isn't important to me and is not what a happy ending is dependent upon.
I could also write about how much I hate Dumbledore and how he was really a cruel, psychopath but I mean then I'd go off on how many other characters I hated. Not to mention Harry was a bit of an asshole as well.
I do have an idea for a fanfic story of my own, but alas, I am a pharmacist and writing and language is not one of my strong points. Perhaps one day.