1. Able to position windows everywhere!
2. ClientStaticID
3. Default Parameters
4.
RedirectPermanent - For letting search engines know file has been moved!
Able to have a bigger URL than 260 characters


5.
6. jQuery is added
once you create
a project
7.
Routing support
8. Ability to set 
meta tags such as 
Description and
Keywords
9. Maintaining
Selection In GridView
<asp:GridView id="grvExample" runat="server" PersistedSelection="true">
</asp:GridView>
<script src="http://ajax.microsoft.com/ajax/jquery/jquery-1.4.2.js" type="text/javascript"></script>
Control over rendered HTML in  FormView and ListView controls
10.
<asp:FormView ID="FormView1" runat="server" RenderTable="false">
11.
ListView enhancement

3.5
<asp:ListView ID="lvPerson" runat="server">
    <LayoutTemplate>
        <asp:PlaceHolder ID="ItemPlaceHolder" runat="server"></asp:PlaceHolder>
    </LayoutTemplate>
    <ItemTemplate>
        <% Eval("LastName")%>
    </ItemTemplate>
</asp:ListView>
4.0
<asp:ListView ID="lvPerson" runat="server">
    <ItemTemplate>
        <% Eval("LastName")%>
    </ItemTemplate>
</asp:ListView>
12. Chart control for charting data
13. New Project Templates
14. Clickability for sending an email / going to a new page
// sending email when clicking
[DataType(DataType.EmailAddress)]
public object Email { get; set; }

// for going to a new page when clicking
[DataType(DataType.Url)]
public object WebURL { get; set; }
15. Improved searching
16. Multi-Targeting
Much better intellisense when targetting older version of asp.net.
17.
Side-By-Side
Can install VS 2010 on a machine with VS 2008, 2005, 2003. 2010 runs completely independent. Can be running 2008 VS with 2010 VS.
1. http://msdn.microsoft.com/en-us/library/s57a598e.aspx

2. http://weblogs.asp.net/scottgu/archive/2009/08/27/multi-targeting-support-vs-2010-and-net-4-series.aspx

3. http://www.4guysfromrolla.com/articles/081110-1.aspx#postadlink

4. http://www.4guysfromrolla.com/articles/012710-1.aspx
Page.MetaDescription = ""
Page.MetaKeywords = ""
18.
JavaScript IntelliSense was improved
Namespaces are now detected
19. 
CSS 2.1 improved standard compliance
Title, leyends, series, chart areas, 
event customization, AJAX, data manipulation (filtering, sorting, etc)
20. QueryExtender Control
<b>Show All Prices Less Than:</b>
$<asp:TextBox ID="txtMaxPrice" runat="server"></asp:TextBox>
<asp:Button ID="btnDisplayProducts" runat="server" Text="Show Products" />

<asp:GridView ID="gvProducts" runat="server" DataSourceID="dsProducts" ...>
   ...
</asp:GridView>

<asp:EntityDataSource ID="dsProducts" runat="server"
   ConnectionString="name=NorthwindEntities"
   DefaultContainerName="NorthwindEntities" EnableFlattening="False"
   EntitySetName="Products" Where="it.UnitPrice <= @UnitPrice">
   <WhereParameters>
      <asp:ControlParameter ControlID="txtMaxPrice" Name="UnitPrice"
         PropertyName="Text" Type="Decimal" />
   </WhereParameters>
</asp:EntityDataSource> 
Author: Daniel Aguayo
Created: 9/7/11
Blog: JustAboutDaniel.com
3.5
Response.Status = "301 Moved Permanently";
 Response.AddHeader("Location", "/");
4.0
HttpResponse.RedirectPermanent Method (String url):
3.5
Response.Status = "301 Moved Permanently";
 Response.AddHeader("Location", "/");
 Response.End();

HttpResponse.RedirectPermanent Method (String url, Boolean endResponse):
4.0
<asp:TextBox ID="txtExample" ClientID="Example" runat="server" 
	ClientIDMode="Static" /> 
public Car(Car car, bool isRed = false, bool isBlue = false)
{
}
void Application_Start(object sender, EventArgs e)
{
    // Call Register Routes method
   RegisterRoutes(RouteTable.Routes);
}

void RegisterRoutes(RouteCollection routes)
{
   // Register routes
   routes.MapPageRoute(
      "Categories",      // Route name
      "Categories/",      // Route URL
      "~/Categories.aspx" // Web page to handle route
   );

   routes.MapPageRoute(
      "Category",               // Route name
      "Categories/{*Name}",  // Route URL
      "~/CategoryProducts.aspx"      // Web page to handle route
   );


   routes.MapPageRoute(
      "Product",           // Route name
      "Products/{Name}", // Route URL
      "~/Product.aspx"      // Web page to handle route
   );
} 
Page.RouteData.Values["Name"] 
<httpRuntime maxRequestPathLength="260" maxQueryStringLength="2048" />
Sources: