Criticisms of ASP.NET

On IIS 6.0 and lower, pages written using different versions of the ASP framework can't share Session State without the use of third-party libraries. This criticism does not apply to ASP.NET and ASP applications running side by side on IIS 7. With IIS 7, modules may be run in an integrated pipeline that allows modules written in any language to be executed for any request.[3]
ASP.NET 2.0 produces markup that passes W3C validation, but it is debatable as to whether this increases accessibility, one of the benefits of a semantic XHTML page + CSS representation. Several controls, such as the Login controls and the Wizard control, use HTML tables for layout by default. Microsoft has now gone some way to solve this problem by releasing the ASP.NET 2.0 CSS Control Adapters, a free add-on that produces compliant accessible XHTML+CSS markup. However, some controls still rely on JavaScript.