Wow, I completely missed your post, aiedail! Sorry about that. I suppose I'll have to make this a watched topic in the future...
Hmm, the perfect piece of fan fiction, huh? Well, one thing I look for at hpff is flow. When you pick up a published book, it always flows nicely and is easy to read. The action, the dialogue, everything is very smooth. There may be boring parts or dramatic parts, but you rarely find choppy writing in a physical book. On hpff, however, there are no rejection letters or "please edit" emails or anything. So it's a big deal to find a story that flows perfectly, that's hard to break away from because there's no point in the book that makes you look away. Flow probably falls in the "happy to read" part of your question.
A solidly good
story takes a little more than flow. It has to have an impressive, memorable cast of characters. Anyone can throw a Hogwarts student on the page, give them a few flaws and a witty friend, and call it quits. But the character drives the story, for me, and I do expect something above and beyond if I'm going to fall in love with it. In the introduction to an Orson Scott Card book, he points out that writing a huge family is a big chunk of work. They can't just blend into the background; each family member needs a single defining trait that the reader remembers them by. In this particular book, there was a violent kid, a motherly one, the eldest, hero-like one, one with metal eyes, and one who blurted out crazy things after long silences. And if you shuffle back to Harry Potter for a second, you'll find the Weasleys are the exact same way: the girl, the best friend, the tricksters, the arrogant one, the dragon one, the Gringotts one.
I feel like I'm regurgitating what everyone else says, though.
So if I were to tell someone how to write a story, I would say these three things: Make me think of something I've never imagined before. Make me fall in love with all
your characters, good and bad and in between. And give me something to think about long after I've turned that last page.
Thanks for dropping by! And sorry for missing this... The Valentine's gifts just keep coming.