Gaga has had a pretty good year (or not, depending on what side of the ARTPOP fence you are) and she might have ended her year on a massive high note. This performance really showed off all of her strengths, her voice, her ability to sing acapella without issue, and her piano skills. It wasn't too weird, in fact it wasn't really weird at all.
Gaga is at home in both restrained and explosive settings, but this shows off a potential for universal appeal. With just a xylophone, a piano, and a great vocal, Gaga managed to blow the roof off of the place, all without alienating any demographics. Grandpa and Grandma aren't going to be flashed by anything, and maybe the lyrics are a bit too sexualized for them, but they will probably be too busy thinking, "Sweet baby Jesus, she has pipes!"
I hope this is a trend to come, this Gaga, is what you should be doing. No more "Koons Balls" by your who-ha-ha, just sitting at the piano, and singing. Scale back to some Fame/Fame Monster level weirdness and you can truly amaze people. Most people are sick of the eggs and the horns, but we loved your American Idol zipper-eye patch weirdness. It's nice to see you're still there, and change is good, but don't change too much on us now.
Overall: A+