The awful (and quite invisible to most people) O(N^2) behaviour of nested/recursive iterators.
I'm quite gutted that they know about it, know how to fix it but it is not viewed as having sufficient priority to merit inclusion.
I work with tree like structures all the time and have to correct otherwise smart people's code when they inadvertently introduce highly expensive operations in this way.
The beauty of "yield foreach' is that the simpler, easier syntax encourages correct, performant code. This is the "pit of success" that I think they should aspire to before adding new features for long term success of the platform.
