Guy:
1) If "I" like a story I wrote, then it's a success. By that measure the few historical fiction stories I have posted here are each a blinding success regardless of their statistics, simply because they are my personal favorites.
This is the writer I want to be. The one who writes for my own pleasure and doesn't worry about whether or not others read it and appreciate it. And I do often re-read my own stories for my own pleasure, so I would say that most of my stories are successful by that measure.
But I'd be lying if I said I didn't crave feedback about how my stories were received.
Guy:
2) If the readers like a story I wrote, then that story is a success. It's often hard for me to predict what readers will like. For example, "Detention With Corporal Punishment" was a bit of a hit here, and that came as a complete surprise to me. I didn't think that there was anything special about that story, still don't. But it's a success!
It's often hard to know if that is true. Some readers are kind enough to leave a comment or mark a story as a favorite, which is obviously a happy sign. And praise from certain commenters whose opinion I have come to value, I appreciate even more.
But otherwise, it can be hard to know if readers liked it. I don't think number of reads is a good indicator, because they may have just opened the story, decided they hated it after the first paragraph, and moved on to something else. Sometimes I'll see in my stats that someone read through one of my serials, which I'll take as an indicator that they liked it (but I may not get to see that if I don't happen to login shortly after they read it) or similarly, if they have opened a number of my stories in a row, with at least a few minutes in between (time to have read them) then I'll assume they liked something about my style, which also makes me feel good.
As to the formula for getting lots of reads -- I think it's a tempting title, as Burgundy, Kyle and Guy have been discussing, along with a relatively short word count. I think people are intimidated by longer stories (and long serials -- not many have stuck with my 30-part serial that I'm just wrapping up!), or pop in when they only have a few minutes to read something fun. Between 1000 and 2000 words seems to be the sweet spot (though shorter stories may also get lots of reads, because after all it's not much of a commitment to read 400 words.)