SMB3 + SMW + wonky motion controls - options to play it your way = mixed bag but still good.
The basic premise is Princess Peach is having a birthday and Koopa Kids and Minnie Me Bowser kidnap the princess and force Mario to trek through the traditional 8 world types of the Mushroom kingdom. There really isn't a story, just a reason to go to an ice world, fire world, pipe world, sand world, sky world, etc yet again for another installment of Mario platforming.
As you can see the creative minds at Nintendo have made sure not to hurt themselves by thinking up new environments and instead focused on making the levels in each one of these staple worlds have new platforming tricks. The big focus is rotating and moving platforms, each with the same flying red and green turtles, and even the different types of 'hammer brothers' introduced with SMB3.
Each world forces you to fight one of the koopa kids, slowly knocking them out of the way. You will have a mid-castle battle with them and also a final castle battle. The levels also have repeatable mini-battles similar to the SMB3 roaming hammer brothers events.
The whole world aspect is very pick-up and play friendly. Once you unlock a world you can travel to and from it at any time via a world selector. After each castle and ghost house you can save the game, and you can quick save at any time and come back right where you left off. The later quick save will lose your place though if you run out of lives before the next save point though.
There are mushroom houses scattered across each world, some do the card flipping game from SMB3 though now it is a punching fence / card flipping game thus more mingled SMB3 and SMW. There are 1up huts, and warp cannons (though you have to find a secret exit in one of the levels to access them). Each level also has 3 star coins placed hidden or in hard to reach places for you to grab.
The star coins are used to unlock special play videos found in Peach's castle. Some show people speed running through the levels, others tricks to get unlimited 1ups, etc. There is a lot of variety. Though they act more as a spoiler to the levels or something to live up to as you hone you skills.
There are a number of power-ups Mario and friends will be able to grab. Mushrooms, fire flowers, ice flowers, mini-mushrooms, stars are all there in their recognizable form. Also added is the propeller hat and penguin suit. The propeller hat acts as a simple way to get a lot of height to a jump and get time to land on tight platforms as you drift slowly to the ground. The penguin suit allows shooting ice just like the ice flower, but also provides a belly slide that lets you smash bricks and enemies along your way. It also negates the foot sliding on the ice levels.
As a single player game it is pretty standard SMB3 and SMW gameplay with items to collect and lots of levels to perform.
The controls is what spoils the game though. The Wii mote is recommended to be held sideways, NES controller style. Those that have Wii motion plus might find they have to keep taking this off or switch to the nunchuck controls. The sideway controls play just like SMB3 with the addition that shaking the remote does a spin jump or uses the propeller hat. Also walking next to a iced enemy or carry-able block requires you to shake it to pick it up. Some remotes will miss read hard button pushes as a shake and then you really have problems. It would have been better, for me, to just have an option to do the shake jump as a double jump instead. Then both camps could have been happy.
Other than moving platforms NSMB Wii adds in local multiplayer. Everyone plays the same map at the same time. If someone gets too far ahead they will drag people along, if more people are behind it will force the person in the lead to hit a wall and stop running. This forces everyone to stay together for the most part or get frustrated.
If someone dies if they have more lives they will appear floating in a bubble on the level. Those alive can pop them and everyone can keep playing. You can also put yourself in a bubble on demand by pushing 1+A. This allows players to skip hard areas as other players progress. This is also a control problem as it is extremely easy to be running to the right and slip onto the A... "Bubbled" - You can also bubble yourself if you are the last person not in a bubble. This forces the level to end and you to start over. "Annoying" hardly describes this problem.
Then there are the technical aspects that frustrate multiplayer gameplay. When someone dies, the game will do a brief pause and the 'dead' sound. The brief pause is unacceptable as it causes timing issues with those still alive probably trying difficult timed jumps and the like. Also if you jump through a bubble it will affect you jump height and distance. This is also unacceptable as now people in the bubble become a hindrance in current players control abilities.
Once someone has lost their last life in a level they will wait for the other to finish the level or die. They are 'awarded' the ability to push buttons on their remote and honk horns in game. Again, this is SUPER annoying. Instead it should be giving electric shocks to the player for touching a remote they just are not qualified to be holding. Once the level is over the person is awarded 5 more lives and the game just continues as normal.
Also, power-up blocks will spew out enough power-ups for every player, but even players that already have that type of power up (or better) can pick up the power-ups. This either makes a break in the gameplay where everyone needs to take positions, or one jerk will end up getting more than his share of powerups. (this comic- http://www.penny-arcade.com/comic/2009/11/20/ -explains it best. Warning: Strong Language.)
There are also, external multiplayer levels and game types where competition is more the theme, like getting more coins than the other players while playing unlocked levels (levels you have opened in the story mode). The multiplayer annoyances described above for these modes now makes more sense. It would have been nice to have the multiplayer team story mode be more friendly and then make these annoying modifications for the competitive parts of the gameplay.
All in all though, platforming is a gaming skill. Those of greatly varying skill levels will have a hard time playing together and because of the annoyances in the co-op mode it will even be frustrating at time for the skilled player to enjoy the game. In short this comic- http://www.myextralife.com/comic/comic-co-op-my-arse/ -pretty much sums up the multiplayer experience.
NSMB Wii is entertaining, it has it's quirks though. Every quirk in the game would be simple to remedy given a simple options screen and given the fact none of these options are given I can't give the game a glowing review. The quirks and annoyances are lazy oversights. Whether it because of lack of play testing, or group dynamics for the test groups, the problem exists the game is not polished and not perfect.
It might be that I tired of the tediousness of basic 2D platformers, with no story, and no real level of difficulty, but NSMB Wii lacks the enjoyment of the classic entries in the series. An experienced player will never run out of lives, and every death will really just be a minor set back in doing the whole level up to that point again. Call it difficult retrying the same jump sequence over and over, I'll just call it tedious as there never is a challenge to just trying again. I said it once, and I'll say it again, if they are going to appeal to the lack of difficulty aspect they need a way to stop unlimited lives spam so overly prevalent in the Mario platforming games.
If you are an addict of SMW or SMB3 and still have a good copy (or even lucky enough to have a copy of Super Mario All Stars) it isn't really worth upgrading to have even more mario with less polish. Then again, having new levels to master, despite the weak control scheme might be worth it. I find a 30 minute burst with my kids is sufficient before I have to leave due to frustration, but as they get better my whole attitude might change and I'll be happy I have a copy.
It is a take it or leave it game, but hardly a required purchase with everything else out there to play.