All about flash
Honestly, it's all about the flash storage here. It's not the CPU or RAM that's making the current MacBook line so fast, but the PCIe based flash storage (the other specs are nice, don't get me wrong, but there not where all that "zippiness" is coming from.)
While a cMBP can get pretty snappy with an SSD upgrade, there's very little left that's appealing about it. Trust me, I held out a long time until I let go of the ethernet, firewire, and optical drive. I've been using a 2012 15" anti-glare MBP as my daily driver with an SSD upgrade, not ready to jump into the new, "Non-Upgradable" waters. But now, after moving to a 2014 MacBook Air, I can tell you I should have done it sooner.
The portability and battery life (OMG the battery life) are worth the trade offs alone, and while it's not retina, I really like the high res of 1440x900 on a 13" display (the classic is only gonna get you 1280x800). The only things to consider with a machine you cannot open up, are Applecare and RAM. Applecare is usually a no brainer to extend the life of this machine. I would also highly recommend jumping to 8GB of RAM. It's true, most folks are fine with 4GB, but it's the one thing you may end up wanting that cannot be added by any means in the future. And at $100 for the upgrade, it's pretty fair.
Other than that, I'm very happy with my new air. It comes with me everywhere, drives my 27" external display, and lasts for the better part of 10-13 hours when I'm out and about. Good luck