.NET 2015 and Visual Studio 2015 introduce significant advancements including the .NET Compiler Platform (Roslyn), which supports multi-platform development and code customization for ASP.NET applications. ASP.NET 5 is a lightweight, open-source platform designed for flexibility and modularity, allowing development across various systems. Additionally, Visual Studio 2015 offers enhanced features like a smart code editor and integration with .NET Core for improved efficiency.