Well, I have to say I am not too pleased with the New Super Mario Bros. games. New Super Mario Bros. Wii was great because it offered new things. The multiplayer and the new costumes made it an improvement on the one for DS (which I didn't like that much). Now they have made the one for DS with a little twist, the twist being mainly that it's easier and pretty much impossible to run out of lives.
The Mario games are still good platformers, but they really need to take some advice and stop calling it New. It's not mcuh different than the last one.
In the past though Nintendo were a lot better at this. There's Super Mario Bros and Super Mario Bros 3 (I don't count 2 because it wasn't released outside Japan) which gave birth to flying, the tanooki suit, the frog suit, the hammer bro suit and all that. It also gave us the first world map in a Super Mario game. It improved the series by far.
Super Mario World came with a graphical update, the ability to replay levels, finding hidden exits and special stages. It changed the system around flying to make it require more tactics, it added the Yoshis. (skipping ahead a bit now) there's Super Mario 64 which offered 3D for the first time which was awesome on its own. It made us collect stars and gave us some puzzles to solve. It really took the series to a whole new place. There's Mario Party which was awesome. Again skipping lots here there's Super Mario Sunshine. This offered a new mechanic compared to the first 3D Mario game spraying water, using water to hover, get rocket boosts to reach new places or super speed. Over to Super Mario Galaxy which also gave us things that were new. Then they seemed to lose their creativity. They made Super Mario Galaxy 2. Also a great game, but built on the exact same premise as the first one.
Mario stopped going places with this. Rather than mixing up the formula they seemed to toss out new games of the same.
Nintendo has been very creative in the past despite using the same characters 80% of the time, but now it seems that they are learning that we'll buy anything they toss out. Virtual console comes out and we buy games we already bought there. They make a new Mario Kart game and we buy that. Now I still like Nintendo games for their quality, but I used to love them for giving me new experiences.