I learned Python back before the 1.5.2 release, so the things that were key for me back then may not be the key things today.
That being said, a crucial thing that took me a little bit to realize, but I now consider crucial: much functionality that other languages would make intrinsic is actually made available by the standard library and the built-ins.
The language itself is small and simple, but until you’re familiar with the built-ins and the “core parts” of the standard library (e.g., nowadays, sys, itertools, collections, copy, …), you’ll be reinventing the wheel over and over. So, the more time you invest in getting familiar with those parts, the smoother your progress will be. Every time you have a task you want to do, that doesn’t seem to be directly supported by the language, first ask yourself: what built-ins or modules in the standard library will make the task much simpler, or even do it all for me? Sometimes there won’t be any, but more often than not you’ll find excellent solutions by proceeding with this mindset.