Over the years, I've had two major productivity issues with Construct since Construct Classic:
1) The inability to reorganize event sheets in the project tree.
2) The inability to easily copy/paste layouts and their contents from one project to another.
Number 1 would make organizing and editing events for large projects much easier. Number 2 would make working with other developers much easier - especially with level designers.
Of course, I'm sure that there was a technical reason these features weren't implemented before, and perhaps they would be challenging to add to Construct 3, but it would be awesome to see them at some point.
Regardless, Construct 3 appears to be shaping into a phenomenal product, and I look forward to playing with the public beta in April! Keep going, Scirra!