Re: The uke/nage paradigm
IME it's important to have continuous feedback about being too compliant/taking a fall on the one hand and being too tense/blocking on the other, sometimes that feedback is verbal other times it's physical.
There's a continuum, at various points along it you have "playing around" between yudansha, being a tense beginner, trying something out as an intermediate, being forced to refine a technique by a senior (the OP point about "providing problems to solve in his or her technique"), etc. There's also being able to take some pretty dynamic ukemi to avoid getting seriously hurt and sometimes that can look a little like being a ragdoll but it doesn't feel that way when you're on the end of it. In theory as observers we shouldn't be any more confident about saying "aiki bunny" than we are about saying "serious aiki"... unless we're in the mix how do we know?