In your opinion, what’s the most important part of a bunny transformation? What parts should get the most detail, focus, etc?
I guess a very common metric for defining what aspects of a transformation to focus on the most would be the degree of difference between the old familiar form and the new one. I'll consider the usual human to non-human as the starting point but obviously if a basset hound TFs into a lop eared type of bunny they would likely notice many other things before the ears ;)
Thoughts about being a bunny below and the actual short form answer near the bottom:
First, the more obvious physical changes. Regardless of length or floppiness, the ears would be one of the first physical changes that the one transforming would notice. The increase in foot size and the softness of the fur beginning to grow on their body would also be a couple of physical traits that would be noticed fairly early. A less immediately noticeable change that would be experienced is the increased heart rate. If the person TFs into a larger rabbit breed they would likely feel their heart rate roughly double... a smaller rabbit breed TF could even double the speed of the larger breed's! Either way, it would seem quite fast and potentially disconcerting to a human that had just transformed.
Next, some of the sensory changes. Those big new ears would open up a brand new auditory world to the person that transformed. A rabbit's hearing is superb and they can detect sounds from far away in every direction. Lop eared rabbits have hearing that's better than a human's but not quite as good as rabbits with upright ears. Rabbits also have an excellent sense of taste with about twice as many taste buds as humans (though they may not be able to taste things as savory). Their sense of smell is also far more developed than a human's (about 15 to 20 times as powerful!) so a whole new world would be opened up in that regard as well.
There may be some behavioral changes as well, some of which would likely be related to being a prey animal, but others that would be related to a rabbit's own particular instincts, such as thumping their foot when they feel in danger or aggravated. The TFed individual may get the urge to rub their chin on friends as this is a way rabbits mark their territory. Similarly, the TFed person might show affection by licking their loved ones, and may softly click their teeth together (which is referred to as purring) when they're very happy.
THE ANSWER: So in regards to the actual TF I would say the most immediately important aspects would be feeling those ears grow, the heightened senses of hearing and smell, the cozy soft fur that would blanket the body, and the new faster heart rate ^u^
Sorry for all the words before the actual answer to your question, but it was the easiest way for me to work out the answer. Hope this has been informative to you for any potential bunnification in your future ^u^