It's thicker and it's heavier.
The only real difference in its build, and it's only very slightly so. The other noticeable change, to my fingers, is the apple logo is now engraved, previously it was embossed. Very slightly, a fraction of a millimetre, but that's where my fingers rest so it's noticeable. It's not heavier like the iPad 3 was compared to it's predecessor; having to carry so much extra power meant a significantly bulkier battery. The retina mini is still far lighter than it's 10 inch counterpart the Air (which, having held, is amazingly light) and is still comfortable to hold with one hand for extended periods. It just lends a more solid feel.
It's faster; iOS 7 looks better. Parallax is smoother, animations are more prompt, there's more transparency, or at least more noticeable transparency. It's got an M7 chip tracking it's motions, a couple of apps have picked up on this but I don't think the M7 is a big deal in an iPad. Better there than not, sure, but not gonna be a 'winning' feature. It will be interesting to see if any apps really make the most of it's inclusion.
The performance is important. The guts of the original mini are approximately the same as the iPad 2. In mobile device years that's geriatric (though to be fair it was performing perfectly well). Yet really this generation is entirely about its title feature: the retina display. Which is stunning. It's the only thing I missed about the full sized iPad, and the longer I used it the more I wanted the definition back - just never enough to contend with the size and weight of the ten-inch tablet.
So this was pretty much the upgrade I'd been hoping for and I couldn't be happier. Reading text at this resolution is phenomenal (compare it to a printed book, the screen is superior), and that's the big deal for me. Sure, games look great, but I don't really have any installed, though maybe that will change. I read and write on here (this blog post in fact), for which there are two key exchanges: fingers and screen, eyes and screen. The experience has gone from good to exceptional. This computer travels with me, and enables me to do just about anything I need to wherever I am. Tethered to my phone it is the most versatile and easy to use computer that I own.