Technically, shouldn't it have been New Super Mario Bros. 2? note Link gives credit to where I first saw this question.
Maybe they're saving that name for when they actually make another game that uses the same formula as Super Mario Bros. 2. (The international Super Mario Bros. 2, I should say. I don't know what they'd call it in Japan.)
The core Mario titles usually don't have numbers. Spinoffs and remakes are another story, but the core titles dropped them a while ago. Super Mario Bros. 4 is Super Mario World, but that makes Super Mario World 2: Yoshi's Island into Super Mario Bros. 5, and that would get really confusing if it was allowed to continue, so Nintendo basically gave up with the numbers. Super Mario Galaxy 2 is an anomaly, but it's also the first Mario game released on the same console as its predecessor since the NES.
NSMB2 has been announced as the title of a 3DS game... huh?
Word of God states that it's considered a "direct" sequel to the original New Super Mario Bros on the DS, by virtue of also being a handheld title on the direct successor of the previous handheld system. Taking this into account, New Super Mario Bros U is likely considered the same for New Super Mario Bros Wii, by virtue of being a title on the successor to the Wii.
How are characters that aren't Mini able to get into the Mini Pipes along with those that are?
Off screen they used bubbles.
I play this game with my younger sister and older brother, and we both cant figure out how the HELL you are supposed to get some of the coins without having 2 or more people. Half of them require one of us to jump into a bottomless pit, then use the A-buttion-bubble at the last minute.
All coins are reachable with only one player. Coins partway down a pit can be reached by throwing shells at them or by using the helicopter hat to jump out of the pit.
Maybe I'm missing something, but is there a reason this game uses Yoshi's Story type physics? I found it sluggish and artificially difficult compared to the crisp, instant-response platforming physics of the old games it was obviously homaging.
Playing Super Mario Bros 3 and Super Mario World before this one, I thought this was easier besides the last two worlds, toad rescues and being less forgiving on ice. Didn't notice much difference either besides a worse spin jump. If the physics really are less responsive you are still able to spin in air and Yoshi can flutter kick. Maybe that's why?