What should I be aware of when moving from asp.net to python for web development?
I second the note by Out Into Space on how python is a language versus a web framework; it's an important observation that underlies pretty much everything you will experience in moving from ASP.NET to Python.
On a similar note, you will also find that the differences in language style and developer community between C#/VB.NET and Python influence the basic approach to developing web frameworks. This would be the same whether you were moving from web frameworks written in java, php, ruby, perl or any other language for that matter.
The old "when you have a hammer, everything looks like a nail" adage really shows in the basic design of the frameworks :-)  Because of this, though, you will find yourself with a few paradigm shifts to make when you substitute that hammer for a screwdriver.
For example, Python web frameworks rely much less on declarative configuration than ASP.NET. Django, for example, has only a single config file that really has only a couple dozen lines (once you strip out the comments :-) ). Similarly, URL configuration and the page lifecycle are quite compact compared to ASP.NET, while being just as powerful. There's more "convention" over configuration (though much less so that Rails), and heavy use of the fact that modules in Python are top-level objects in the language... not everything has to be a class. This cuts down on the amount of code involved, and makes the application flow highly readable. 
As Out Into Space mentioned, zope's page templates are "somewhat" similar to ASP.NET master page, but not exactly. Django also offers page templates that inherit from each other, and they work very well, but not if you're trying to use them like an ASP.NET template.
There also isn't a tradition of user controls in Python web frameworks a la .NET. The configuration machinery, request/response process indirection, handler complexity, and code-library size is just not part of the feel that python developers have for their toolset.
We all argue that you can build the same web application, with probably less code, and more easily debuggable/maintainable using pythonic-tools :-) The main benefit here being that you also get to take advantage of the python language, and a pythonic framework, which is what makes python developers happy to go to work in the morning. YMMV, of course.
All of which to say, you'll find you can do everything you've always done, just differently. Whether or not the differences please or frustrate you will determine if a python web framework is the right tool for you in the long run.
I'm thinking about converting an app from Asp.net to python. I would like to know: what are the key comparisons to be aware of when moving a asp.net app to python(insert framework)?
Does python have user controls? Master pages?