Wii Fit Plus has everything Wii Fit has, and some new stuff.
It has some more exercises in every category.
It has some more balance games, like the birdman one (you flap your arms to fly from cliff to cliff, landing on targets Pilotwings style), the butt-math one (you hit numbers with your butt and hips to reach a certain sum), and a platformer one (you run and jump over some cliffs).
But the main new feature is that you can customize an entire workout. You can select several exercises, put them in the order you want, and start the whole thing. In the original Wii Fit, you have to pick an exercise, do it, go back to the menu, pick another exercise, do it, go back to the menu. So half your workout is clicking buttons in between exercises. I'd much rather just press one button and do my whole workout, everything in a row.
To me, this one new feature is worth the purchase and the price difference. But new exercises and new games is awesome too.