Why is Grand Theft Auto: Chinatown Wars so impossibly fun? There's a pretty substantial technical achievement in cramming a big, open, no-load city onto a DS cartridge. And the art design team deserves serious kudos for making areas of the city look and feel significantly different despite the tight constraints of a more or less 2-D design. But I'd say the basic system-level strength is that the minigames are fun.
For most hardcore gamers, the sweetest words are "Grand Theft Auto game", and the bitterest are "minigame collection", so it's a little blasphemous to say that's pretty much what GTA is. But it is---and that's always been its strength. In any GTA game, you spend a lot of time taking missions, most of which are just drive-here-shoot-him. But what's always most memorable are the missions that change the rules for a little while---the jetpack in San Andreas, or the sniper runs in Vice City. GTA IV didn't have as many rule-shifting missions, which is much of why GTA IV got---ulp!---a little dull in the middle.
GTA:CW tells you right up front that you won't just be driving and shooting when your character, upon arriving in America, is immediately kidnapped, and is stuck in a car that's dumped into the river. You quickly tap the windshield to bust your way out---nothing fancy, or even particularly entertaining, but a straightforward warning that you're going to have to keep your stylus handy. This would be a pretty serious strike against the game---holding the stylus while using the thumb-pad and buttons is kind of a pain in the ass. But it's justified by the number of times the game shifts context on you---forces you to twist wires quickly to steal a car, tap numbers to bypass a security system, twirl a Chinese dragon costume, and so on.
Combined with GTA's usual attention-deficit-disorder approach to level design---"now steal a car now drive to Hey! let's be a cabbie! drive to your destination and collect your Hey! Let's help this guy bug his wife's car! follow the car until it parks don't get spotted while you Hey! Let's buy some acid!"---these stylus-based minigames don't have to be very challenging to accomplish their basic goal, which is to keep you from ever settling into a unitary rhythm of play. Instead you're always a little surprised, never sure what you're going to be doing in the next 10 minutes.
That's why Chinatown Wars has me hunched until my neck snaps, while Far Cry 2 is languishing in my 360. Far Cry 2 has a lovely open world, a solid story, and some very nice mechanics. But it's all driving and shooting, and after playing for 2 hours, I felt like I had done pretty much everything I was going to do in the game and now just had to do it another hundred times. Besides a reasonable level of wit and some solid visuals, GTA remains the king of the unexpected mission parameter, and that will keep me playing to the end.