Dave de Vos
I've been practising aikido for a little over three years. If my instructor would give me such a compliment, he would probably mean that my attack is well aimed and well controlled and has the desired amount of energy and that my response to nage's technique was appropriate as in correctly anticipating atemi and falling / rolling safely. These are the things I'm aiming for when I'm uke and perhaps your sensei meant something along those lines.
However, on a more advanced level, I think a good uke knows exactly how much of a challenge his attack should offer for a particular nage. And ideally he should be able to make nage aware of any mistake he makes in a way that allows nage to figure out how to fix his mistake. But this level of ukemi is beyond my current ability. I think that in general, it takes more than three years to reach this advanced level of ukemi. I would not expect many students to have this ability, but I'd hope that the instructor and some advanced students would have it.
Thank you for your clarification. Being an uke is actually fun, except for some painful locks.
The ukemi-learning process takes long, that is something everybody says. One of the people in our dojo said once: "I practice Aikido for 25 years and even I can't still take ukemi perfectly." It is a learning process. In the old times, first you were only thrown around for like 2 or 3 years and then you were able to fall properly.