I personally don't mind mat buttons over glossy ones. I DO mind glossy controllers though. In fact I mind anything glossy that you have to hold in your hands all the time, since they're fingerprints magnets, and they hand up looking all dirty just from being touched.
As far as buttons go, the only thing I've been minding from Nintendo in general is how tiny they've made their D-pads to be on pratty much any of their hardware in the past decade. I can understand on handhelds meant to remain compact (although that still doesn't make for very comfortable controls), but on something like the DSi XL or 3DS XL, there should definitely be a larger D-Pad on there! The D-Pad seems larger on the Wii U Controller Pro though, kudos to them for that but the whole controller is a tad too bulky for me to play, say... a Street Fighter game on the D-Pad of the thing. But for anything "twin-stick" and using the shoulder buttons, that controller is ACE.
I used to play MK8 using the Wii U Pro Controller because it's more convenient for me (lasts longer on battery than the gamepad itself anyway) but the buttons are a bit too... how should I put it... HIGH, like they're too thick and small, so it leaves a mark in my right thumb after a while from holding the A button down constantly. So I now play using the Club Nintendo SNES controller for Wiimote that you could exchange your stars for on the European Club Nintendo at one point. And I play MK8 using the comfortable D-Pad of the SNES controller, and I'm right at home with that one.
Nintendo controllers and handhelds that I hated the D-Pad on because it was too small include the DS Lite, DSi, DSi XL, Wiimote, Gamecube Controller (not to mention the placemet on that one, but few games really made you want to play with the D-Pad anyway), 3DS, 3DS XL... and I can only imagine it's going to be super tiny on the New 3DS and New 3DS XL too... The worst is the 3DS one... because it's squeaky and on mine, plastic on top of it peels off, literally... Not the cheapest D-Pad I've encountered, but we're close.