Fun stuff
Well Nintendo does deliver somewhat on two of these things. The story is still the same except it's held in an alternate universe where Luigi is the hero. But playing as Luigi, and the new character of nabbit, does make the experience different.
Gameplay:
The changes make gameplay slightly different, but it is the same old mario bros platforming. Luigi feels very luigish, which means he is a more tricky character, considering the longer jumps but skiddy physics.
The levels are more difficult, with a 100 second time limit. While it makes the experience fun, there is a bad side to this; the levels are surprisingly short, and time limits don't give you the time to wander or mess around. Time is never the most fun thing to fight against, and having the time finish just as you are about to end the level is frustrating.
Value:
This game costs 20 dollars. Now, do you get your bang for the buck here? Well, for 83 redesigned levels, a different experience, and considering the crap value of some other DLC's out there, I'd say it's a fair deal.
Multiplayer:
Nevertheless, to get the full value, you definitely need to play with friends. The game is easier alone, and while you won't quite breeze through (it's quite hard actually), you can still finish the whole thing in a relatively short amount of time.
One thing big N could have done better was to make Toad slightly different from Luigi, but they are more or less the same. Still, Nabbit is a nice change, and while it doesn't make multiplayer worth it, it is a step in the right direction.
Sure, this is game isn't much of a change from NSMBU, but it's enough of a change to make the experience fresh and worth getting, especially for 20 dollars.