Here are a few of the quick google searches:
1) Access to customer data:
ASP.NET 3.5: there is no direct access to data from the client side. We can go for any of these ASP.NET 4.0 methods: this structure has a built-in function for this. Below are methods for their implementation.
2) Meta keyword setting and meta description:
Meta keywords and descriptions are really useful for getting a list on a search engine.
ASP.NET 3.5:
ASP.NET 4.0: , .
3) Enableviewstage
ASP.NET 3.5: : "True" "false"
ASP.NET 4.0: ViewStateMode , : Enabled, Disabled Inherit.
4)
ClientID .
ASP.NET 3.5: ClientID, ,
ASP.NET 4.0: ClientIDMode ASP.NET.
: http://www.dotnetspider.com/resources/34755-Main-Differences-between-ASP-NET-ASP-NET.aspx