In Gnome Shell interface when viewing Activies, we can scroll the window to zoom in and zoom out. Can this be done in Unity 3D?
You should always provide information about which versions you are using. There are significant differences between Unity in 11.04 and Unity in 12.04, for instance.
In this particular instance, it isn't important, since you can't do this in any of them. However, they are working on a complete replacement for this, which hopefully will land in 12.10. You might be interested in reading up on the current ideas: http://design.canonical.com/2012/03/task-switching-in-ubuntu-and-a-introduction-to-the-spread/
You might also be interested in learning how to contribute to the design. The lead designer, John Lea, wrote a post about that, which you can find here: http://design.canonical.com/2012/02/the-unity-design-process-and-how-you-can-play-a-part-in-it/
The complete Task Switching specification can be found here: https://docs.google.com/document/d/1EdrlUuZvA9P8-BZufUU2KlHGjg49p9UacF4MCL0U5uA/edit?pli=1
If you would like to get your ideas in, this is the time to do it. Well, after the holidays, anyway. :)