Gave this thing a go with just a little bit of expectation, World's Remaster was so underwhelming it killed the hype I had for this game. But color me surprised, this game lived up to the initial expectations!
The game is well polished and faithful to the original experience as far as gameplay goes. You want SMB1 as it is on the NES? You got it. It also has a lot of options and features you can customize to your liking, like level layout tweaks, back scrolling and modern damage, so you can make it easier on yourself or keep it 100% purist if you want.
For the main modes you have...
Super Mario Bros, which is self explanatory.
Lost Levels, with it's respective visual changes and special Worlds from 9 to D (as the achievement for them implies) and... it seems Luigi plays the same as Mario.
Super Mario Bros Special which is the standard SMB1 experience with Special's level design and an unique visual/music presentation. The hammer is there too and Stars have smiley faces!
All-Night Nippon, which is a beast of it's own. You play the regular SMB1 levels under a combo meter, with a level design that accommodates for holding on your streak. Visuals and aesthetics from All-Night Nippon are also present, while adding special effects the higher your combo goes and when you get a Star, mainly colorful spotlights and a cheering crowd. All-Night Nippon focuses on finishing the levels with STYLE!
You also have Deluxe's Challenge Hunt where you go through levels hunting for Red Coins, Score and Yoshi Eggs. You vs Boo is here too! And these challenges are available for all modes except All-Night Nippon.
So yeah, this might be the definitive way to play Super Mario Bros! Give it a go, it's very fun!