Hard limits must always be established and respected. Period.
That said, I'll quote Bob Dylan, "I used to care, but things have changed." For those of us who are long-term in the life-style, our tastes and some, or maybe many, limits have evolved. As a sub, there are many things that were initially in the NFW column, but having extended relationships with skillful, experienced Dommes in whom I developed total trust, most of my early hard limits are no longer.
#1 was trust, followed closely by communication (or the reverse order). Understanding the much wider range of how certain forms of play can occur, testing out something with a clear understanding that either of us could bail out at any time, and, frankly, reading about others' experience here have all contributed to a much larger and broader palette.
As a Domme, communication and understanding (never assume you really understand without checking more than one way) are paramount. Many years ago a sub was ONLY interested in enemas - - fine. NO SPANKING, she said - - OK. A year or so later, she saw my paddle collection in my bag and admitted some curiosity. A mild demonstration. Next time, a bit more, etc. Soon enough, that hard limit was completely gone because of communication and trust and a willing curiosity. But her limit was always respected and only she could change it.
I totally agree that there are some immutable hard limits: children, animals, deadly risks. I've bled a few times, but those were accidents or incidental and not intended or carelessly caused. I would never ask for that, while respecting those who do. I ended a relationship with a sub who sought more damage (spanking, whipping, caning) than I was willing to administer. In a few cases, there are activities I would only try as a sub in the hands of a known expert, such as fire play, bullwhips and similar high-skill fetishes.
RACK and SSC are real and important. So is knowing what you're doing - on both ends, so you know when it's going/gone wrong. Venturing into new activities demands doing your homework, understanding anatomy, physiology, signs of medical trouble, knowing your counterpart's medical circumstances, etc. Don't try something new or adventurous if either one of you is tired, or intoxicated or dehydrated. Be smart. Pay attention. Listen. Being responsible means exactly that - every time.
For me, some of the hottest experiences have come from pushed limits - on each side - when creativity met trust and care.