What works about the game? Well, I feel like I've already given you that answer – side-scrolling Mario is great. Saving the Princess is great. Beating Bowser is great. It is a beautiful formula and Nintendo has taken to heart the adage of "if it ain't broke, don't fix it." There is some new stuff here—like the aforementioned coin counting—and some new Mario power-ups like a gold flower that lets you shoot fireballs which destroy enemies and change bricks into coins. But, mostly it's just Mario being Mario and Bowser being Bowser.
Graphically speaking, the plumber and the levels look beautiful in 3D. The 3D isn't oversold, but it does add some pop and this isn't one of those games which looks 10 times better with the 3D off. I will say that I want to see the game again on the Wii U and a big screen, but it looks good on the 3DS and no one who plays it is going to have any complaints on that score.
So, to recap, Mario is back in side-scrolling glory, and he's got to save the Princess from Bowser. If you liked it before, you'll like it again. If you didn't, you won't.
New Super Marios Bros. 2 is rated E (Everyone) by the ESRB for Comic Mischief.