The most exciting release in SharePoint history has been introduced to the general community at SharePoint Conference 2009. The product has host of new features around scalability, maintainability, social and all and all a great enterprise product. Sneak peak vidoes give good overview about the investments in Developer and Infrastructure space. There are many features which I wanted to write on but I am not sure what's under NDA so just highlighting few which are applicable to any SharePoint deployment. They are:

1. Developer Dashboard - Interactive way in which develoeprs can see what's happening on their pages. Why they are slow and they can get a trace on page itself about the time taken by different webparts to render themselves. Support has been added in base classes for spitting out this information during different events in page lifecycle. This will help to easily isloate the problem areas for slow rendering pages.
2. Powershell support - PS is now first class citizen for managing the SharePoint Farms. Lot of scripts have been written by the Product team to automate the management of various activities in a farm.
3. Scalability aspects - Service model has been introduced for various services which were earlier part of SSP. Now each service can be hosted individaully and managed in individual databases. This help in isolating the content, manage backup/restores and overall make the system robust
4. Logging - New concept of co-relation guids have been introduced which log each message for a call in logs with a guid that can be used to correlate the messagge with the error. The corelation guid is shown on page when error occurs. This can be used to find the related messages.
5. Business Connectivity Services (BCS) - This one is most exciting feature in v4. All those who have worked with BDC in MOSS2007 have enjoyed the capabilities of the integeration feature for fetching data and for search. This version renamed to BCS goes a step further and provide symmetric programming model on client as well. What this means is that you can connect to LOB systems using the BCS Api's even from client software like Outlook, SharePoint Worksapce, Excel etc. You have read/write capabilities and there are far more connectors now including new addition of WCF and .NET custom assemblies as out of box connectors. It also allows you to write your own custome connectors. That's a great capability and I am sure you will see some great apps made on this framework.

There are many more features which you will see people blogging and documented on official SharePoint sites. Get started and start playing with the bits. You can download the bits and get rolling. Here are some links to start with

Developer Content - http://msdn.microsoft.com/en-us/sharepoint/ee514561.aspx

IT Professional - http://technet.microsoft.com/en-us/default.aspx