We started the development of a new project at work, complately ASP.NET driven.
At first glance, ASP.NET looks great, but it still has some minor issues.

Like working together on 1 project makes debugging difficult, one can only debug once at the same time. This is of course not really an ASP.NET issue, but rather a Visual Studie .NET, which we use in development.
Another thing is the designer which randomly changes my HTML code, I'm not a big fan of WYSIWYG editors, and especially not the ones that change my HTML code. Thank god we don't have to use this one all the time.
It also tends to change code. For example, when you have a title tag with the runat attribute, for changing it programatically, and you paste it in another file, Visual Studio removes the runat attribute. Don't ask me why, but it does.