First of, let's get one thing straight about sex & women - what scares one woman in bed might make another woman all hot and bothered, so you never can tell how she's going to react to a certain kink, fetish or unusual practice -- until you ask her, of course.
The kinds of women who are into these things probably aren't going to shy away from talking about them. On the other hand, if you bring it up and she bolts, at least you'll find out early on that you're sexually incompatible.
So here are few things that you should definitely not try without at least talking about it first:
New territory The first thing that could scare a woman in bed is asking her to do something that she's never done before. Depending on how confident she is, a request for something she has no previous experience with may have her shaking in her boots (or stilettos, as the case may be). Before you spring a new idea on her, get a feel for her knowledge and comfort level on the subject. A simple: "Have you tried (insert intimidating sexual act here) before?" can go a long way to not scaring her before you get started.
Choking her Wrapping your hands around her neck and giving it a little squeeze might be perfectly OK if she's been warned ahead of time that it's something that turns you on, or certainly if she is the one that requested it. However, choking her out of the blue during sex is a surefire way to scare her in bed. Many people genuinely enjoy rough sex, violent role-playing and even some real pain, but if you haven't discussed your predilections with your sexual partner well in advance of getting into bed with her, you must not spring something like this on her while getting it on.
Unexpected anal play Some women are turned on by the idea of anal stimulation, while others find it disgusting. Before you know which kind of woman the one you're in bed with is, do not go anywhere near her ass. Unexpected anal penetration is pretty much guaranteed to scare her in bed. Even getting too close to the general area with your fingers, your tongue or a toy can be enough to freak out some women. Get an idea of her views on the act before you try anything anal.
Extreme sex props Battery-powered sex toys and handcuffs may be a normal part of your sex life, but if you're with a new partner who hasn't yet checked out your toy box, don't whip these things out while you're in bed with her. Most women would be able to handle a simple vibrator or maybe a blindfold, but if your idea of fun props includes shackles, ropes, whips, and textured butt plugs, you may want to ease her into the idea before you bring out all your toys. Safe sex toys like the ones from Loveplugs are probably best if you're introducing props for the first time. Remember that the most important thing is that you are both comfortable. If you're up to the level of prostate milking sticks, Electotorture or speculums, keep this stuff well hidden until you know for sure that she's cool with your fetishes.
Psycho dirty talk Most of us can appreciate a little dirty talk in the bedroom. In the heat of the moment, all kinds of things can come out of your mouth if you're not careful. The downside of this is that actions aren't the only things that can scare her in bed; your words can do the job just as well. Maybe you started off by whispering a fantasy in her ear, but once your voice takes on a serial killer-like intensity and you start getting into too much psycho detail, your little story takes a dark turn and she makes up some excuse about having to wash her hair or get up early in the morning. Keep these demented little tales to yourself if you want to avoid scaring her in bed.