This new feature for 1.5 brought a huge smile to my face.
You know when you've mostly built a scene, then you realize you need to change a few things early in the scene? You shift all the activities on the timeline and that's when you find that all your carefully set up timings are now just ever so slightly off and it's immensely frustrating to get them all back again.
With the new timeline grouping feature, you can CTRL-click on several activities at once and move them all at the same time. Voila! Your timings are preserved. No more fiddly adjusting everything back to how it was.
That's going to be a huge time-saver.