By far the biggest improvement in SharePoint 2010, in regards to this new release of our product, is the ability to better control the asynchronous ItemAdded event receiver. Given the positive feedback at the SharePoint 2010 conference when Microsoft announced the addition of such a synchronous ItemAdded event, quite a number of people must have encountered that problem.
Recap: in SharePoint 2007, the ItemAdded event fires asynchonously and so does the thread that brings up the EditForm.aspx with the properties of the newly created item. This of course creates a collision if both threads then try to modify that same item. The only way out of this issue was to provide a custom EditForm.aspx page that would wait for the ItemAdded event receiver to finish before the EditForm.aspx would load and access the item.
In SharePoint 2010 the ItemAdded event receiver can now be configured to either fire synchronously or asynchronously and also be given a sequence number. This eliminates the need for a custom EditForm and any manual synchronization and allowed a much "cleaner" solution with the new product release.
On the user interface front, the Ribbon does take a bit to get used to, both for programmers and users. I do think though that accessing the Ribbon is easier for many users than having to search through several popup menus for a certain command.
The ability to have client-side ECMAscript code and the new Notification elements in the user interface is definitely a great step forward. We made use of it for the manual metadata extraction that now happens asynchronously at the server while the user can continue to work in the browser.