Hmm - well I still feel relatively n00b as a 3k. For me the most prominent ones were
15/16k: I think that's the moment I started evaluating different moves instead of just playing what looked best and hoping I pick better ones than my opponent.
12k: discover first bits of wholeboard thinking
9k: discover first vague ideas around direction of play
6k: discover tenuki / sacrificing locally for global benefit, leaning attacks
6k-4k: reading, basic haengma (moving with shape and speed)
4k-3k: life&death, reading
3k-: I'm approaching 2k mainly by making my group strong before attacking, attacking to push around not kill