I was absolutely in love with the Game Boy Advance and thought it was the best handheld ever created. I got the DS (original, fat, ugly gray) within a week of launch, and played EVERY meaningful game for the system up till late 2008. You could say I burned out of handheld gaming pretty hard, seeing as I've had Game Boys of all variants since like 1995 or something. I don't own a Game Boy Advance or a single game for it anymore, nor do I have a DS. I sold them all because I was so totally done with them.
 
So although the 3DS looks very promising and great on so many levels, there are a lot of things holding me back from jumping on the bandwagon anytime soon. I love the N64 updated games like Zelda and Star Fox, the new entries like Super Mario, and ports like Street Fighter IV and Metal Gear Solid 3, but I've also been burned pretty bad by Nintendo's stupid prerogative with the Wii and all those lame casual sensitivities and the lack of decent online features.
 
SO, here's my list of things that will get me into the 3DS for sure.

• A revision or fix for the battery life issue (5 hours?!? Are you joking?)
  
• A couple serious killer apps (Super Mario looks like it could be cool.)
  
• Bigger screens - the touch screen is smaller than the one on the DSi? WHAT? I don't need DSi XL proportions, but decent big screens are a MUST.
  
• Solid lineup of downloadable games (along the lines of the XBLA and PSN)
  
• GBA games added to the virtual console (specifically: Metroid, Mario & Luigi, WarioWare, Advance Wars, Castlevania)
• Any sign that Nintendo is going to allow DLC for existing games, plus patches and updates
• Achievements or something like that would be cool, especially if there was a point to them (like in-game bonuses, special Club Nintendo products, etc.)
  
• Metroid Dread. Done.
• Also, Picross 3D 3D. Make it happen. Super Smash Bros. would be tight too, maybe.
  
• Price drop. $250 is way too steep for what you're getting. I'd gladly just take a DS2 without the 3D for less. As it stands, I could get a PS3 for less, so...
• UPDATE: I have heard reports that emulation of old DS games is fairly poor, and that's bullcrap. A revision would have to fix that.