Python vs C#/.NET -- what are the key differences to consider for using one to develop a large web application?
I would also suggest we must compare runtimes and not limit to language features before making such moves. Python runs via interpreter CPython where C# runs on CLR in their default implementations. 
Multitasking is very important in any large scale project; .NET can easily handle this via threads... and also it can take benefits of worker processes in IIS (ASP.NET). CPython doesn't offer true threading capabilities because of GIL...a lock that every thread has to acquire before executing any code, for true multitasking you have to use multiple processes.
When we host ASP.NET application on IIS on single worker process, ASP.NET can still take advantage of threading to serve multiple web requests simultaneously on different cores where CPython depends on multiple work processes to achieve parallel computing on different cores. 
All of this leads to a big question, how we are going to host Python/Django app on windows. We all know forking process on windows is much more costly than Linux. So ideally to host Python/Django app; best environment would be Linux rather than windows.
If you choose Python, the right environment to developed and host Python would be Linux...and if you are like me coming from windows, choosing Python would introduce new learning curve of Linux as well...although is not very hard these days...
My organization currently delivers a web application primarily based on a SQL Server 2005/2008 back end, a framework of Java models/controllers, and ColdFusion-based views.  We have decided to transition to a newer framework and after internal explorations and mini projects have narrowed the choice down to between Python and C#/.NET.
Let me start off by saying that I realize either technology will work great, and am looking for the key differentiators (and associated pros and cons)  These languages have a lot in common, and a lot not--I'm looking for your take on their key differences.
Example tradeoff/differentiator I am looking for:
While it seems you can accomplish more with less code and be more creative with Python, since .NET is more structured it may be easier to take over understanding and modifying code somebody else wrote.
Some extra information that may be useful:
Our engineering team is about 20 large and we work in small teams of 5-7 of which we rotate people in and out frequently.  We work on code that somebody else wrote as much as we write new code.
With python we would go the Django route, and with .NET we would go with MVC2.  Our servers are windows servers running IIS.
Some things we like about ColdFusion include that its very easy to work with queries and that we can "hot-deploy" fixes to our webservers without having to reboot them or interrupt anybody on them.

I've read some of the other X vs Y threads involving these two languages and found them very helpful, but would like to put Python head-to-head against .Net directly.  Thanks in advance for letting me tap your experiences for this difficult question!