*shifty*

*delurks*
As a lurker who is a joiner of many boards but poster of few... I've realized that there are several factors which affect whether or not I review a story. Quality, length, status/update frequency, pairing and author. Also of course, whether or not I have time but there's nothing a writer can do about that. Since I post quite infrequently though, I try to make my comments count for something the best that I can.
About the factors:
Quality - I think this is self explanatory. Good stories are more likely to get reviewed because if you don't have anything good to say about a story, its best you don't post just to flame. Constructive criticism is another thing but that probably takes more brain cells than an encouraging message and lurkers in general are lazy (at least I am).

I do take into consideration that quality plot/story telling is not the same as quality spelling and grammar. The former is definitely more important but only by a small amount. I can find reading material with perfect grammar easily but the same cannot be said about desirable plot. Well written work deserves recognition so that's a tick in the 'to review' box.
Length - In general I don't review drabbles because in all likelihood my review will end up longer than it. I think a drabble is more for a writer to get out a scene or an emotion that has the potential to later on evolve into something more substantial. Without building upon it though. Well done drabbles with good build up and conclusions within the constraints of the word limit are very rare. I prefer stories of 1000 words and up. With multi-chapters stories I feel more invested in the characters and being able to give predictions of what may be ahead of them is more of an incentive to review than if it's only to say I liked it. Longer stories also have more themes and plot to analyze and comment on.
Status/Update frequency - I prefer to read finished stories because I tend to be nervous about whether or not a story will be finished or not. However, I more consistently review the few ongoing stories I do read. This is probably because of the belief that I need to review so that the author knows people are following the story and are motivated to continue. Stories that update faster will also more likely get reviews because my interest to follow the story is better if I can actually remember what happened the last couple of chapters before. I tend to forget about fics that haven't been updated after awhile - unless they're exceptional.
Pairing - Very big factor for me but maybe minor for others. Especially for rare pairings, reading these make me extremely motivated to write a review and maybe add in a comment for 'MORE'.
Author - Things such as author comments and replies factor into it too. Authors who comment too much on their own story within the story text (such as with parenthesis) are a deterrent for me, simply because you don't want to 'talk' to someone who keeps interrupting you/your reading experience. One I'm not particularly proud of is that when an author demands a certain number of reviews before releasing the next chapter, it's just part of my personality to automatically rebel against this sort of 'threat' so I often won't post a review as if to say 'pfft, I can live without your story'... although I might cave in instances where it turns out I can't.

Ah, but really I guess the most important thing is a fic you really, really, really, really, really like makes you want to review it because you want to encourage more of the same.
@Estrea: I haven't read a lot of your H!P fic yet but I've read some of your Nanoha stories on ff.net... I probably didn't review so I'll take this chance to say I really enjoyed "Ribbons".
