I think one of the juiciest features of 4.5.X is the addition of the new Add_to_cart_popup module. It is the best of both worlds. Traditionally you have to choose whether you want to redirect to cart and possibly lose out on additional impulse buying, or keep them on page with the ajax cart update and potentially confuse your visitor about whether the item is added or not. I have seen most of the bigger brands out there adopt this popup method. It also gives you an option for upselling.
So what do users with 4.4.X do? Upgrade their highly customized carts? No longer!
I have been contracted to backport this module to a 4.4.4 site, and I am going to track my progress and steps in this thread. Stay tuned!