I haven't thought of anything that my guy has definitely said no to, but there are things that I want to try that he's uninterested in or iffy about, including...
-spanking implements other than his hand. Although now that I've seen the aftermath of some nastier cropping sessions and such, I'm iffy about more serious sessions with such implements. I'm ok with sticking to barehanded, I just want to try other supplies to know whether I'd be able to handle them.
-collars that I like. My guy likes this rhinestone choker thing, but I like the more basic look of plain metal collars (like the steel circlets or even the patterned wire collars on BondageCollars). I showed him some heart-shaped tag things with extra charm things (you can get them engraved)... but while he said he liked them, I don't think he got that they could be collars... I'll wait to bring it up again until he's in one of his really dom-my moods again.
Those are the main ones. He has also suggested some more humiliating stuff that sounds really hot, but he says they can be punishments if I give him attitude. But they sound sooooooooooo fun and I NEVER want to give him attitude.
He's also a little more into the threesome idea than I am I think. So that's one where it works the other way around, where he's interested and I'm iffy. But we are willing to do without or wait until we're both comfortable with an idea.