Faster CPU (though I don't have a link to benchmarks), vastly improved integrated graphics, faster SSD, thinner/lighter, better battery life, Thunderbolt 2 (vs. TB version 1).
I bought a new mid-range model for my daughter today (2.4GHz, 8GB RAM, 256GB SSD) from my local Apple store and didn't want to hand it over to her after installing her apps and other configuring. So, so very nice.