It's not the only killer feature. Where Python 2 lacks consistency Python 3 fixes it. Where Python 2 core libs have stagnated, the Python 3 ones have been improved. All future development from core CPython developers is going into Python 3. That's a pretty killer feature.
Slow, incremental improvement is not a killer feature. Compiled Python 10x faster. That would be killer. Proper multi-core concurrency. Killer. Modest (yet breaking) improvement over 6 years while all the action goes on in other languages? Not killer.
But you cite a very important point: all the improvement, modest as it is for the majority of users, is going to 3.x and yes, that's why I've moved. But I can tell you it's only because of the constant nagging and threats about abandoning 2.x. Nobody would have moved for any actual "feature" of 3 were it not for the fear of being abandoned. It's a stick-only strategy. No carrot.
In my book, these are only killer features compared to Python 2, not in comparison to the rest of the world, and more along the lines of minimum necessary to justify the pains of a breaking change.
