6.5? Come on!
You'll notice that a lot of the gameplay is the same. Kicking Koopas, stomping on Goombas, all that good fun that was in the original Super Mario Bros. But the levels are longer, harder, a bit more interesting, and quite different. A lot of the levels have noticeably different (and sometimes tricky) layouts, and some weird stuff, such as a strong wind, Bloopers and Cheep Cheeps in non-underwater levels, and a Poison Mushroom that can power you down, or kill you. This is certainly different from what we're used to, but makes for a unique experience, IMO. Also, this game has more than eight worlds this time around. I don't want to spoil much, but you'll notice that it continues after 8-4.
Also, interestingly, you can choose to play as either Mario or Luigi, rather than just Mario. You basically select "Mario game" or "Luigi game". Luigi is slightly faster, and has a higher jump. At least to my experience.
There's not a lot to say about the graphics and sound. They really didn't improve on it a lot, save for slightly better graphics, and maybe a couple of new tunes. Now, sure, the graphics and sound from the original SMB were awesome, but they could've at least tried to improve on it a little more. Ah, well. Not a terribly big deal.
Overall, pretty awesome. My only real complaint is that it can be pretty hard at times (and is harder than the original in general). But for what it's worth, it's another awesome Mario game, and definitely worth playing.