Having just now gotten 5 (sparkly) stars on my save file I think I can give my final opinion.
Best New Super Mario Bros. ever.
As I said in my previous comment, it's a shame this game is dunked in the "New" vat, but when you come to terms with that (seemingly a long process for some) you really start to appreciate the things that this game does differently from others in the series.
A big plus is my book is the added focus on exploration. Some coins are pretty deviously hidden and it took some real effort to find all of them. Likewise the bonus levels. In NSMB 1 you just had to defeat bosses with mini Mario, here you have to actively seek out alternate exits and there are quite a few of those. This is a dense game and just like in Skyward Sword coins/ rupees are hidden absolutely everywhere. There are lots of trigger points in every nook and cranny and even though you always get relatively meaningless coins it's aways fun to seek out these little secret areas.
The absolute biggest pro I can give the game is in its astonishing diversity. I was constantly amazed by how unique almost every level is. Almost no asset is reused. There are lots and lots of levels where the structure (or level gimmick) is completely unique and is never repeated again. I was always very excited to unlock new levels this way, because they always have me guessing at what comes next. Again, the reuse of the "New" assets may diminish this feeling for some; you've probably seen some concepts used in previous games.
So then the coin mechanic. Here's where they fail. The actual coin mechanic is actually used quite well I think. As I said, every nook and cranny potentially holds hidden coins and it's a lot of fun to find out where they're hidden and trying to grab as many coins as you can never realy gets old. Even if you don't care for this, you can just ignore it and the game won't punish you for it at all, remove it entirely and the game works just as well. That is good game design right there. However where they blow it is in the coin's worth or lack thereof. Why collect coins if you can literally do nothing with them or even get rewarded for them. Maybe they can add something using DLC, but as it stands they made the coin collecting feature nigh useless.
Also Nintendo: change the fucking music.
That said, the variety and density of the levels make up for most downsides. NSMB1 remains the black sheep of the series with its uninspired level-design, NSMBWii was way better and had great level design, but I truly believe NSMB2 finally reaches the series' full potential.
Well I don't know wtf then.
I'm sure I never used the suit, the internet is telling me if I did the level would remain red on the map and I don't have any of those..
EDIT: And just got all the rainbow worlds, I'm out of idea's.
You're sure no one else has access to your savegame? My nephew screwed me this way on Super Mario 3D Land.