Globally unique identifiers
It is possible to write software that detects duplicate feeds, but comparing two articles is not an inexpensive operation, and when comparing a very large number of feeds, the O(N^2) runtime is painful. I have experimented with much a less accurate algorithm: hash NGRAMs of articles and check for duplication using a hash lookup. I have found that this gives poor results - at least in my experiments. If you do partial matching of NGRAMs, you are back to O(N^2). (If anyone knows a good way to handle this, let me know :-)
Globally unique identifiers help solve many duplication problems, makes it easier to implement container relations, and in general ATOM just seems to be a better and more scalable platform than RSS 2.0 for complex new applications.