When you put 'New' in the name of something, it's inevitable that around a decade later someone will smarmily say "it's not new any more", and you just have to accept that putting an adjective like that in a name isn't a long term solution. Yet, when New Super Mario Bros. was released on DS it was entirely appropriate and succeeded in creating a lot of excitement - in May 2006, believe it or not, getting a new 2D Mario game was a special, rare event.
Brushing aside spin-offs, ports (or remasters, however you want to regard the 'Advance' games) there was a lengthy gap between bona-fide new 2D Mario platformers. The Nintendo 64 revolutionised platforming with Super Mario 64, and the GameCube stuck with 3D Mario with Super Mario Sunshine. Portables saw re-jigged ports with new and exclusive content, but it can be argued that Super Mario World was - prior to NSMB - the last proper and new 2D Mario game (if we treat Super Mario World 2: Yoshi's Island primarily as a Yoshi game). By the time 2006 rolled around, it shouldn't be underestimated how keen Mario fans were for a back-to-basics 2D platformer with the podgy plumber as the lead character.
The New branding, so easy to mock now, was actually a smart move. As the DS took off and started to sell in huge numbers Nintendo was establishing a vast audience of both veteran and fresh fans, so New Super Mario Bros. was a name that intrigued long-term gamers that had waited very patiently for a 2D Mario game, and also made very clear to recent fans of the company that this wasn't yet another port of a classic. With a brand new visual style that utilised 3D assets on a 2D plane - which could be considered 2.5D, let's not quibble over that right now - it looked rather impressive at the time and showed off what the DS could do. It was a pitch to create new Mario memories for a modern era - blending nostalgia with change.
In terms of gameplay, being on the DS it naturally uses the touch screen a little, and even throws in plenty of mini-games. Mario's moveset is tweaked compared to the 'Bit' days, and new and familiar enemies have a fresh look. It's also accessible in a way that's now become Nintendo's standard approach; it's relatively easy to get to the end credits, but finding and accessing all of the secrets needs a little extra effort.
As this debut 'New' entry turns 10 years old we can reflect on the fact it made fresh 2D Mario games part of the norm again. Each system since has had its own game - New Super Mario Bros. Wii, New Super Mario Bros. 2 on 3DS, New Super Mario Bros. U, the latter two even including DLC. New Super Luigi U, for our money, is well worth a look for anyone that's skipped it so far.
There was undoubtedly a point, particularly in 2011/2012, when Nintendo's releases and announcements in the series bunched up and there was a bit of over-saturation. That should have passed by now, though it's unclear whether Super Mario Maker has changed the game and Nintendo's approach to 2D Mario releases. Perhaps the next game will ditch the 'New' branding altogether for a tweaked marketing approach; only time will tell. It's important to remember that New Super Mario Bros. on DS was a hugely significant release, however, and has been an incredible success. Nintendo's latest figures show it to be the biggest-selling DS game by a mile, shifting a sensational 30.8 million copies.
It's not 'New' any more, but it's an important part of Mario's history.