Not the best Mario game by far, but it's still a great game.
The basic concept and design has barely changed, but level design in brimming with new ideas. There are some new power-ups, including the propeller, which lets you soar into the air for a short amount of time. It sounds cool, and it is, but it's also a cheat in my opinion- it ruins many of the star coins.
The next new power-up is ice mario. This lets you freeze enemies then chuck their frozen bodies at other enemies. This is a good power-up, though the good ol' fire flower beats it by a mile. Lastly there's penguin. This is merely ice power with a few extra abilities. So it's not really new. The fire flower also returns, good news for any mario fan.
One of the biggest additions to New super mario bros wii is multiplayer. This is great, chaotic fun due to players getting in the way of each other. While this may sound bad, in reality, it just increases the challenge. But while someone powers up, gets iy or dues, the screen freezes for a split second. So if you're making a jump when that happens, the momentary freeze will de-sync your jump, probably cause you to plummet to your doom. This results in largely broken multiplayer.
The graphics and sound of NSMBwii are good, but hardly great. The music is the same sort of bubbly, predictable, but catchy tracks you heard in New Super Mario Bros, while the graphics are a bit disappointing, even for a wii game.
Level design, meanwhile, is the mario we've come to know. Every single level is created with skill, offering a decent challenge and throwing in many different mechanics, though I was disapointed in the lack of many uses for the wiimotes motion sensing.
The bosses are the same ones from mario world- the koopa kids. Each has a twist that changes it from an average boss fight to something special. Every single boss is different and well thought out.
In conclusion, NSMB wii is a fantastic game, and one of the best 2D platformers on the wii. It does have flaws- mainly broken multiplayer and lacklustre visuals and music- but overall, it's a challenging game any fan of platforming games will enjoy.