.NET has a lot there in its web framework out of the box, so you if you know it well you can do sophisticated things very quickly. There is even strong javascript/ajax integration these days, and new MVC patterns. Overall, I also think C# is a very strong, modern language with tons of benefits.
However, the issue comes when trying to find off the shelf components. Unlike with Python and Ruby where there are tons of OSS projects you can integrate, with .NET you will have to either build everything on your own or pay a LOT of money for components.
Another thins to keep in mind is that working in .NET requires VS and all the suite of .NET profilers/debuggers which are not free like they are for Python and Ruby. Therefore when growing your dev team you will have to pay a lot in software per seat.
However, the issue comes when trying to find off the shelf components. Unlike with Python and Ruby where there are tons of OSS projects you can integrate, with .NET you will have to either build everything on your own or pay a LOT of money for components.
Another thins to keep in mind is that working in .NET requires VS and all the suite of .NET profilers/debuggers which are not free like they are for Python and Ruby. Therefore when growing your dev team you will have to pay a lot in software per seat.