If you talking about browser compat. Your server platform choice does not absolve you of handling differences in browsers rendering, so this has nothing to do with choice of server side platform. All server side technologies I know of
articles to a site using ASP.NET. So as a client you could care less if it is PHP or ASP its all abstracted away by the time it reachs you.
Not in my experience.
"Pure" ASP.NET is great, but it's when you get into System.Web.UI.WebControls (rather than HtmlControls) where you enter client dependencies.
Modern additions to ASP.NET like Control Adapters and Atlas/AJAX muddy the waters even more and make it harder to have a "clean" project.
I also miss VS2003's Web Application project model (VS2005 SP1's still doesn't support WebDAV). VS2003 was also faster at Remote Debugging.