There are a lot of differences between CRM versions CRM 4 and CRM 2011. It can take you a while to get use to the changes but after an initial period of confusion I think you will find the changes have dramatically improved developing CRM extensions.
The biggets improvement is Microsoft getting rid of the CRM variables, they are a complete pain and means you have to spend hours converting everything. I also think the solutions concept is brilliant.
I haven’t mentioned the new functionality but focused on what has changed in simliar functionality. The new functionality is
Webresouces,OData, WCF instead of webservices, online version allow sandboxed plugins, SSRS reporting using FetchXML, Sharepoint intergration
Synatx Difference between CRM4.0 and CRM2011
Data Type Changes:
Java Script Changes:
Custom Workflow Changes: