Why I can't get into New Super Mario Bros.

[Never]

Indeed. Look how different super mario land 2 (1992) was to super mario world (1991).

They both had overworld maps but sml2 did it differently and was less linear in the order you could do things.
Almost every baddy and boss was new and unique in sml2.
The level theme types were completly different.
The level designs tended to had a different structure and style of design. Less levels that were simply about moving from right to left and jumping over pipes. In it's place new environment types and obsicales to maneuver around with more up and down movement.

Both great games but had their own character. SML2 was like the devs wanted to make a game that was still a mario game but diffrent to SMW in every way.

NSMB games don't try to do that. I would say that when there was only one NSMB game it did have it's own identity and was great. It gave me hope for 2d Mario. It borrowed from the past but did lots of new things too. But now we have NSMB Wii/2/U as well. Despite many tweaks and little changes, they feel like expansion packs, and the series seems lazy and uninspired.

2D Mario could easily be revived in my eye's if Nintendo wanted to.

I'd love to see new settings and baddies and sprite base graphical style. Maybe throw in some galaxy style gravity tricks. Actively avoid what the previous games did whilst still have the key ingreadiants of a mario game.

I disagree with you on 3d world though. I see that as an evolution of the galaxy games with some compromises made to make it suitable for multiplayer. The only reason you might say it lacks its own identity is because we had 3d land before it. If they released a 3rd 3d world esk game you might start to have a point.