Andy787 is off his rocker, couldn't disagree more with him...
The system
Cheap/affordable
Touch screen, stereo / virtual surround, built in microphone.
The design isn't the best, but the clamshell setup is tidy - it flips open and locks into positions exactly how you'd want it to and it protects the screen.
The bay at the back of the console means its hard to lose a stylus.
If the stylus isn't big enough for your own hands there are larger (sometimes extendable) ones available from third parties ie. the battle stylus
The touch screen is very responsive, you'll never need to use the system's built in calibration
Pictochat is a lot of fun, even restricted to the same room. Future (online) versions, if they happen, should be a blast.
The games:
Mario 64 has better character models, better animation and replicates the gameplay well in the handheld form. The minigames are genuinely fun.
Mario 64: there are three different control methods you can switch to on the fly by pressing select. You can use the stylus/thumb strap (and the d-pad/buttons handle jumping/punching etc, with triggers acting as Z). You can use normal d-pad and buttons.
Wario Ware: Touched! -- some feel it's not as good as Wario Ware: Twisted... that doesn't mean it's not fucking awesome.
Zoo Keeper / Polarium / Meteos -- even the worst of the three is a fun distraction, DS is (and will continue to be) a great game for puzzle games. Look to Bust a Move DS.
Single Cart multiplayer -- seems to be far more supported than it is on PSP, although I'm sure it will happen to some extent there too. A great way of testing games and spreading words about others... or just having a good ol' handheld multiplayer affordably.
The online lineup - dig up the tentative list of online games on Nintendo.com... if you don't have one, buy a router in preperation.
Nintendogs - puppies!!
New Super Mario Bros - first new 2d mario in years, it looks brilliant
Mario Kart DS - 30+ tracks old and new, single player battle mode with bots for the first time, and online multiplayer
Metroid Prime: Hunters - genuinely great FPS control, and some of the best graphics on DS - it's gonna be great to see how far it's come along
Viewtiful Joe: Scratch - looking good!
Advance Wars DS - Advance Wars fans are looking foreward to it. If you don't know what all the fuss is about yet, maybe you should investigate?
Animal Crossing DS - as above
Pokemon Pearl
|_ Third party:
Guilty Gear,
Lost In Blue,
Tony Hawks American Wasteland (online),
Ultimate Brain/Card games,
Bomberman DS (single card multiplayer),
Nanostray,
A Boy and His Blob,
Meteos,
Castlevania,
Bust A Move,
Black and White Creatures,
Sonic Rush,
Snowboard Kids,
Top Spin 2 DS,
Age of Empires 2,
Bubble Bobble Revolution DS,
Boktai DS,
Baiten Kaitos series,
Dynasty Warriors DS,
Final Fantasy Crystal Chronicles,
Final Fantasy III DS,
Dig Dug Digging Strike,
Monopoly DS,
Top Gun DS, etc
Backwards compatability
Great GBA playback on either the top or bottom screen. I'll leave how great some of the games are in the GBA library to other people / threads.