<p>I have a web application using ASP.NET 2.0 and I want to know if I should be moving it to ASP.NET 3.5, particularly... what am I missing by not moving to ASP.NET 3.5?</p>

<p>I understand the disadvantages, but I don't understand the advantages.</p>

<p>What are the biggest benefits of migrating/rewriting? </p>

<p>Will I get a speed improvement?</p>

<p>Is MVC that much easier than my old-fashioned WebForm application?</p>

<p>Will it look cooler?</p>

You will only miss access to the newer .NET 3.5 libraries, and cool syntax such as LINQ and lambda expressions. Performance wise they will run the same.
By the way, ASP.NET MVC is NOT included with .NET 3.5...yet.