I can't really share in any detail, I think, but the best part was that "Series" were immutable and had sorted keys (indexes). Essentially they were (math) functions, so "indexes" had unique elements. All the important bits had fast numpy/Cython implementations, but the semantics were good because of unique keys.

Honestly I still feel like I'm missing some sort of larger story about the semantics of Pandas (like the "functions" explanation above), so if anyone knows of anything that made Pandas click, please let me know.