1. Implicit Variable Creation
This is probably my biggest complaint. Why would you want to implicitly create a variable the first time something is assigned to it? This might make sense in a language where variables are immutable by default (after all, you only ever assign a variable once), but Python isn't. Also, I realize there is a technical difference between rebinding names and changing the value of a variable; I don't find that particular distinction useful here.
2. Dearth of Collections (in the standard library)
Don't get me wrong: defaultdict and namedtuple are nice, but on occasion I really find myself wishing for some more advanced data structures, e.g., Red-Black tree. I'm not even talking about probabilistic structures like Bloom filters or skip lists.
3. Lack of Tail-call Elimination
I know it likely won't happen, but I still wish I had it. To me (and I'm sure many others), recursive algorithms are the most natural way to express certain algorithms, e.g., traversing a tree. I can do it using a loop, but it really drops me out of the zone.
4. Concurrency in the Standard Library
In an ideal world, Python would support concurrency on a level with first-class functions, similar to Erlang. It's almost not even fair to ding Python on this, since pretty much every other language its age has the same problem, but a man can dream, right? At least the multiprocessing module made it into the standard library.
5. Interfaces
It would be really swell if Python had support for something like interfaces. I know that PEP 3119 introduced Abstract Base Classes, so this one is probably on the way to being remedied, but the feature is so new I haven't yet encountered it in the wild.
Back to flipping out...
