Android lollipop has come with alot of improvements and changes to the overall quality design and performance to android, windows 10 should go the same route, new features( not delete& copy options)we should see significant change in the UI, APPS & functionality of the OS.
Oh yes I do like the new Material Design(in Lollipop) but I dont think it was a big change, Google is just trying to implement uniform UI so that new/redesigned apps will follow a standard(like for Metro UI for WP but how many apps in WP do we see with the same Design pattern?
), performance boosts is under hood, and yes, ART is supposed to be more power efficient and faster than the Dalvin. But our WP OS is already lagfree and mostly power-efficient . I would agree with you that WP10 should bring out extra features, more APIs for developers and better functionality. Lets see, what happens.
Whatever happens, it's upto developers to bring their apps better. A OS redesign(as you think) may not bring an evolution in Apps. The WP is still 5% or whatever market share.