Interesting question. As I am such a beginner, with most practises I have no idea whether I would like them or not. The list 'never tried' hugely outweighs the list 'we tried it'. Also, currently I am away from my boyfriend and can't try anything (long-distance D/s does not really appeal to me). What I would not want to try:

Piercing. In fantasy, the idea of chaining someone up by a ring in his body puts a huge grin on my face. But whenever I see a real piercing, it completely turns me off. It seems that I only like the abstract concept, and dislike the way it looks. You are making me wonder with how many other things that would be the case, too!

Branding. A branding scene in a story can be hot [sic]. But in reality a brand would, in my perception, spoil the beauty. I also suspect that I would dislike giving that much pain. The notion of 'marking one's property' is meaningless to me.

Fisting. I like reading scenes of guys who get fisted. But even with lots of lube, I would be too doubtful about long-term health consequences to try it in reality.

Fanciful titles. Respect and politeness are a huge turn-on for me. But much of the 'verbal elevation' I like in fiction and fantasy, if applied in a real situation, would merely make me collapse with laughter.