I've been playing this game off and on for the last week or so. Considering that I am a massive Mario whore and that the last two proper Mario 2D platformers were Super Mario World and Super Mario Land 2 in the early 90s, I don't want to rush through this thing. It's been over half my life since then!
I try not to ruin too many surprises by paying too much attention to early game media, so my expectations for New Super Mario Bros. were essentially Mario platforming most similar to the first Super Mario Bros., done well, plus a few of his newer tricks like wall jumping. Thankfully those expectations have been passed. Considering that it's been so long since the games mentioned last paragraph, every time I see some new thing not found in the old games it's very strange. It just gives a sense of "Wow. It really is a sequel with all new stuff!" Enemies that punch through blocks. Switches that deform the territory. Piranha plants on conveyor belts. Levels full of springy mushrooms, or mushrooms that sway from side-to-side. Or levels that don't particularly use new items, but are just well-designed and novel, like a level taking place on moving, intersecting floating platforms where sometimes piranha plants are dropped off or picked up from the player's platform. Or a level where you're essentially on an elevator ride up through a castle, and spiky spheres bust through the wall. Sometimes they pass right through and bust through another breakable wall, sometimes they'll bounce back and forth and you have to try to deal with it. Just fun stuff.
The new moves from the 3D Mario games integrate fairly well. The butt stomp (which, after all, began in the 2D Yoshi's Island) makes for some interesting reversals. Rather than jumping up to break a brick block or get a powerup from a ? block, sometimes you must butt stomp from above. The wall jump is great; one of my favorite things about Super Mario Sunshine was how it was able to be used repeatedly, and such is the case here. Given a several-blocks-wide gap, you can shimmy right up. The double and triple jump thus far seem less useful. In most circumstances there's just so much stuff in a level that you're not going to have a very clear area for running and jumping in such evenly spaced amounts.
New powerups. Giant Mario is... interesting. Grow huge and bash up the level a bit. It's a bit like Even More Invincible I guess. Koopa Mario seems like an SMB3 powerup, in that it's a pretty interesting idea, but it's uses are pretty specific. Mario wears a shell, so he can run and slide around like a shell you've just kicked. Tiny Mario is quite cool, though. It's like after all the years of not having a regular Mario and a Super Mario, they're overcompensating by allowing for an even smaller version. Tiny Mario dies when hit rather than reverting to regular Mario. He can reach certain areas through tiny pathways larger Marios can't fit in, and he can run on water!
One minor gameplay annoyance... things can't be thrown upwards. In Super Mario World if Mario is holding a shell, you hold up on the d-pad causing him to look up, and release the hold, he'll throw upward. In New Super Mario Bros. he just remains staring forward, and throws it forward. I'm just not sure what was such the big deal about throwing upward that they decided not to allow it.
The polygonal graphics do the job. Certainly at these resolutions a sprite look could be cleaner, but that wouldn't allow for nearly the smoothness of animation, rotation, or scaling that this game takes advantage of. And I'm sure I'll be even more thankful when the game scales well in some higher-resolution rerelease a decade down the road.
The music is a particularly weak aspect. There are a few catchy tunes, but this part doesn't stack up to the oldies nearly as well as other aspects. Though I'm sure nostalgia plays some role in this.
Overall... it seems an impossible task for this game to approach the major status of the older entrenched Marios in my mind, but it seems to at least be trying quite well.