For someone who’s been happily programming in C# for quite some time now and planning to learn a new language I find the Python community more closely knit than many others.  
Personally dynamic typing puts me off, but I am fascinated by the way the Python community rallies around it. There are a lot of other things I expect I would miss in Python (LINQ, expression trees, etc.)  
What are the good things about Python that developers love? Stuff that’ll excite me more than C#.
I'm an asymmetrical user of both languages, in a sense that I use C# mostly professionally and Python for all my "fun" projects (not that work is never fun, but... you know...)
This difference of context may skew my perspective, including my opinion that they are two distinct types (pun intended) of languages for, generally,  distinct purposes.
This said, it may not be a coincidence that Python is, at this point in time, [one of?] the languages of choice for all kinds of cutting edge, somewhat scholarly, technology/science oriented projects.  (And BTW, this "scholarly" keyword here doesNOT imply, that Python is a university toy, plenty of "serious" applications in plenty of domains/industry are proof to the contrary).  This may be due to several factors:
(I don't develop most points, readily well expressed in other responses)

the openness and quasi universal availability of Python (unlike C# !)
the lightweight / ease of use / low learning curve
the extensive, high quality, "standard" library and the extensiver (and occasionally bum quality, but on the whole available, open-sourced, etc.) additional library.
the wide array of open source projects in Python language
the relative ease to bind with C/C++ for reusing legacy code, but also for placing performance-critical portions of a project
the generally higher level of abstraction of may constructs of the language
the multi-paradigms (imperative, object oriented and functional)
the availability of practitioners in so many domain of science and technology

and, yes, the

"herd mentality effect" mentioned in a remark, possibly in a [self?] deriding way. The fact that a language attracts a broad, "closely knit" community, makes it attractive too, beyond the superficial ("look cool" and such) traits of herd mentality. Put in broader context, sometimes the best technology/language to use is not measured on the its intrinsic merits but on the overall "picture", including the user community.
What Python features will excite the interest of a C# developer?