As Shopping is becoming more personalized, so do the customer's demands. Add to Cart does not suffice anymore. Go more in depth with 3 extra rules:
1) If customers re-order an item show them Reorder An Item instead of simply Add to Cart. You can even throw in a discount for them if they reorder a particular item.
2) If customer already has items, show them Add Another Item because their cart aleady has items. Show them that you understand which part of the checkout process they are currently in.
3) Checkout everywhere - Add a convenient header button to check out from any screen and at any moment. Make it prominent and easy to order.
Cart Improvements are what should be better called Cart Common Sense. Almost no Shopify theme has it. Be the one who really is the expert and understands your customer's needs and individuality.