If you're referring to .NET's UpdatePanels, they do send the entire post data, yes. But the response only contains required HTML to rerender that area.
ASP.NET can also take any "web service" (i.e., a bunch of function you define and mark as directly callable via XML or whatever), and VS will generate JavaScript proxy objects for it. Then you get lightweight calls (only send and receive parameters and response), but get IntelliSense and stuff on them.
Hotmail might do something stupid and be slow. I don't know or care, but .NET offers plenty of options.
ASP.NET can also take any "web service" (i.e., a bunch of function you define and mark as directly callable via XML or whatever), and VS will generate JavaScript proxy objects for it. Then you get lightweight calls (only send and receive parameters and response), but get IntelliSense and stuff on them.
Hotmail might do something stupid and be slow. I don't know or care, but .NET offers plenty of options.