Forenote: In the spirit of this thread and this board I'm in no way condemning any of the authors or writers who use any of the below listed items, I appreciate all of the contributions to this board regardless of whether they satisfy my specific niche desires.
I'm another of the limited number of transformation fans here so I'll start by saying that I skip pretty much any story that doesn't have a fembot with a human origin. I'm not disgusted by it or disapprove, but it does nothing for me.
For me it's all about the human and their struggle with becoming a fembot/cyborg/robot, if it's built from scratch the main draw is absent.
The exaggerated breast enlargement is a bit of a turn off. The jury is still out on the milk producing scenes.
I've also come to realize that people I wouldn't be attracted to in real life don't make for very compelling fembots. That goes for preteens just as much as it does for the middle age crowd. Slutty prostitutes, homely folks, etc. It's much easier to become emotionally invested if I'm attracted to them, even if they're transformed into something the author considers to be more appealling. Yes, I've come to accept I'm shallow.
Really bad technology tends to be a turn off because it's a distraction. I'm not an engineer in the least, but at least have a plausible explanation for the functionings of your creation if that's the route you're going.
Conversely, I can get distracted by technology and explanations that are too well thought out, but that's usually because I'm impressed by the author's research/imagination in creating something fresh. In that case, I'm happy to be distracted by storytellers that are intelligently pushing the boundaries of our fiction library.
Similarly, "classic" technology is also a turn-off. I get nothing from gears and pullies. The "key in the back" is usually a cue for me to exit the story.
Specific to transformation stories, I often prefer to see something more original than "a needle full of nanobots" being the only thing that is used to transform a subject. Ditto for magic.
Bimbo fembots are not particularly attractive to me.
I'll also echo others who've stated that stories can be very sexy without having to have elongated or for that matter any sex scenes. Passion can certainly be created without having to actually detail the act itself. Having a character's lust restrained has been used well in many stories I've read.
Hmmm... yeah, that's enough enemies for one post.