I can sum up my thoughts on the Nintendo 2DS pretty simply: Do you have a 3DS? Yes? Then it's not for you.
It does lead me to an interesting set of thoughts, however. The 2DS is somewhat similar to the Wii U's Gamepad: Slate, controls on either side of the touchscreen, etc. It's almost indicative of a new design direction, though I suspect the next-generation handheld from Nintendo may either stick with the clamshell design, or go all out on the tablet front. The latter's looking more likely, as there's enough pressure from Xbox's SmartGlass and PS Vita's Remote Play to justify a move in that direction.
The weird part, though, is that I've taken stock of all of the gadgets that I keep on accumulating, and I see a few simple trends come up. Everything's got the same family of chips inside (ARM, often a PowerVR graphics chip,) a touchscreen, a gyroscope, and so on.
I'm not going to get into a holy war over who makes the better portable device, but people do tend to simplify, and if these gaming systems are struggling to keep up with our smartphones, then what's the next step? I'm guessing that the phone is going to get involved soon.
On iOS, you have the iPhone and the iPod Touch. Granted, these make for expensive gaming consoles, but they do just about everything else better than their gaming-specific counterparts. Web browsing, location apps, Netflix, music playback and management, it's all simpler on the iOS front. Add in the AppleTV, and we're just waiting for Apple to release some games for it, giving you an instant wireless touchscreen controller. With the upcoming iOS 7 adding native support for game controllers, this would be a huge entry into the market, and Apple's already shown better chops in digital distribution and online multiplayer than Nintendo has.
On Android, you have countless Android phones, the Google Play setup, and Chromecast. Once again, two screens, controllers are optional, and the pieces are just sitting there, with some assembly required.
Microsoft will probably be content to turn your Windows Phone or a potential Surface Mini into the same exact thing, and Sony and Nintendo are already there.
I think Nintendo has a way to go before worrying, of course: They have a rich first-party library of games. You can't just fade away overnight when Mario, Zelda, and Pokémon basically exist to print money. You also can't compete with a $129 handheld designed to play those games: Nobody is buying a $199 iPod Touch for their six-year-old, after all.
I'll admit to being a little worried about the idea that Nintendo is going to be limited to the children's market for upcoming consoles, but it's a comfortable niche, and one that the other companies aren't touching. Where's the kid-friendly Vita? The kid-friendly Surface? The kid-friendly iPod? While $129 isn't cheap, it's definitely cheaper.
My best guess? Smartphone and tablet games will continue to grow in the tween-through-adult markets, Sony will enjoy a small-yet-loyal Vita fanbase, and Nintendo will continue releasing platforms that support their games until Apple comes around with a $100 iPod Touch. Even then, Nintendo will go a little crazy, releasing their own niche-y handhelds with features that only make sense for gaming, just like they have with 3D displays, stereoscopic cameras, analog pads/sticks, and so on.
I'm personally looking forward to the possibility of the 2DS Micro: With some aggressive design, a higher-resolution display along the lines of the old Game Boy Micro, and the almost-unthinkable possibility of no physical media outside of an SD card, they could make a pocketable tablet-style system with 3DS compatibility. If they DO toss in the 3D screen, as gimmicky as it is, they can basically charge whatever they want and get the hardcore market. Doubly so if they make it look like an old Game & Watch.