How much back story?

Before I even started writing, while I found myself interested in back story that an author didn't choose to share, I never felt the need to complain about it in a review. However, now that I'm seriously writing, reviewing, and publishing, I find myself getting irritated at readers who mention back story in a review in a negative way. There seems to be a lot of "not enough back story" or readers finding stuff in a story implausible because they didn't get enough back story in their opinion. I think- like with computers where you can have "user error"- you can get "reader error." When I first started thinking on this, I thought of Alta Hensley's "Traditional Love." A domestic discipline story in which a woman goes through a divorce and moves into her best friend's brother's house. As the story unfolds, a romance blossoms between them. Now please scroll until you get to Anastasia Vitsky's review of the book; she decided to take the time to talk about the speed of the romance in her review of the book. But really... so these characters are written as knowing each other most of their lives- if I'm remembering the story right (it's been awhile and quite a few books since I enjoyed the book myself)- why can't readers think "hm... they've known each other a long time. Do I really need all sorts of back story to make this romance believable?" After all, in telling a story, the story teller (whether writing or telling the story aloud) has to pick a point to start. While "playing in the sprinklers on a summer day as six-year-olds" might be part of why a pair of lovers care for each other, does it really need to be in a romance novel?

So I'm not going to go read the reviews of "Out of the Night: Book One" just so I can quote at you from them, but yes, I've gotten complaints (largely from non-BDSM readers) about back story. I think one that made me laugh the hardest was a reader who accepted me to show Mearr/Vala at a 12 step meeting. Um, yes, the book isn't porn- there is a plot, a story line- but taking her to a 12 step meeting wasn't a necessary part of the story, not something I was interested in writing.

Now when Vala joins The Queen's stable, he already has 18 slaves. How much of their individual stories, how much Vala would reasonably learn (since except for specific parts, it is mostly in her point of view), those were all things that I considered as I was writing and continue to consider through edits. I had a reason for my choices.

Okay, vent done. So I wasn't venting so much with Alta's book (buy and read it if you enjoy a good Domestic Discipline/spanking book with heterosexual characters), but well about Vala's story, I became venty :D. Time to write something else lol.