Okay, I just played, and beat, Metroid on the 3DS.
Here's my thoughts:
Visuals: This may be the best the game has ever looked. I mean, yes, it's an 25 year old 8-bit NES game, so it looks like what it is, BUT the visual fidelity on the 3DS console is extremely good. The colors are very vibrant and appropriately saturated. There's been minimal flickering (still apparently near doors). The sprites are very crisp and the contrast between the darks and lights very good. Maybe it's just the fact that my original NES copy had a ton of flicker and looked washed out on that old standard TV back in the day, but I don't even recall the Gamecube Prime tie-in looking this good.
Music: It all sounds like classic Metroid. While the speakers are appropriate, they're still rather middling compared to a good stereo system, so if you can, try and play the game with earphones on. It makes Ridley and Kraid's lairs so much more menacing and intense.
Performance: Okay, it's been awhile since I played this game... so can someone tell me if the prior versions had a TON of slowdown? This game became barely playable for a large portion of it. When several enemies showed up, the game became a slideshow. It threw me off timing my jumps on platforms and shooting enemies; it was very, very bad. I can't remember if the NES or subsequent ports were this bad, but several times I died simply because the game chugged to the point of single-digit frames. I know the NES had some slowdown, but my most recent memories of the Gamecube version were that it played much smoother than this.
I had the game crash on me once (kinda). An enemy knocked me into a wall and I got stuck. I had to reset the game. Then, for the life of me, I could not get the Energy Tank in Kraid's boss room (you shoot the wall and roll off the ledge holding right... and it never worked).
Maybe I've just been looking at the old game through rose-tinted glasses, or the new games are just so much better, but I don't recall the original game performing this badly. Can anybody speak up and comment on this so I know whether or not I'm crazy or if this is a similar problem for others?
New Features: It's the original game, through and through. All the old passwords. All the same secret. All the same enemies. It's a faithful port. Even the credits remain unchanged. There is one pleasant addition, which is the ability to put the system in sleep mode and pick up where you left off. You can even quit the game and play other games and come back to it. It's a very, very convenient feature to have. I wish they had included Restore Points (I'm told those may come later), which would ease the difficulty curve a bit, and I DESPERATELY wish they had used the bottom screen to augment the classic game with a map. Seriously, this game is great and all, but the original suffered from a huge, winding, confusing world. It needed a map, and it still needs one badly, and that empty, dead screen at the bottom would have been perfect for it.
Other comments: I didn't mind the 3DS layout at first... but after playing for a few hours, it really hurts your hands. It's just not very comfortable to hold at ALL, and that's a problem I've found with nearly every game on it. That's more a comment on the 3DS itself, but for a game like Metroid that requires lots of quick action, button presses, and pressing the Select button to swap weapons, it's worth pointing out. It's difficult pulling these off, especially after long sessions. I got rather frustrated trying to sway ice beams for missiles when fighting Metroids and the select button wasn't responding or I had trouble finding it quickly since it's not poking out or easy to press. They honestly should have allowed the option of rearranging the controls and perhaps using some of the more readily accessible unused buttons instead. That final stand-off against Mother Brain and the escape? Ow. Gave me hand cramps.
So, there we have it. Vintage Metroid, on the 3DS. Not a bad port, with strong visuals, but I found the rest to be rather underwhelming. The slowdown was horrible for me, and I can't help but feel that something, anything, could have gone a long way towards making the original game be more enjoyable to play, whether it was adopting Restore Points or a map or even the ability to arrange button controls. Oh well.