3DS:
• Outstanding first-party games (Kid Icarus, Zelda, Pokemon, Fire Emblem)
• Great third-party games (Monster Hunter & Ace Attorney are the most noteworthy)
• Wide variety of smaller digital games (Pushmo, Mutant Mudds, HarmoKnight, Dillon, Sakura Samurai)
• Virtual Console for classics I never played before (Metroid II, Shantae, Mega Mans)
• DS backwards-compatibility
• Inexpensive
• Good battery life (mine came with a Nyko pack; I get at least 6 hours out of it)
• Miiverse (the ability to take screenshots of Ace Attorney, Zelda, and Fire Emblem is great)
• Multiplayer (by the end of today practically all of my closest friends will own a 3DS, despite not all of them being primarily Nintendo gamers)
I think that covers everything.