The iPhone's most prominent feature is of course the touch screen. While the Nintendo DS also possesses such a feature, it also contains a standard set of buttons. The best DS games seem to be ones that utilize one or the other, with many mediocre games ending up as a painful mishmash of ideas. You get the sense the developer of these games really wanted to use the touch screen but perhaps didn't know the proper way to integrate it into their game.
So now the iPhone comes onto the scene without any buttons at all. This also creates the interesting issue for seasoned gamers of there being no feedback whatsoever. I've seen a number of games devote a portion of the screen to being a controller, however I've always felt this a sloppy implementation. Just touching a bit of the screen to shoot or move forward or any other action feels almost artificial. Likewise I feel using the iPhone's tilt mechanisms for almost anything feels clumsy and uninviting, plus you can end up looking pretty goofy if you play it in public.
So when presented with making a game for this device you end up with a lot of strange ports of games that perhaps shouldn't be on any portable, nevermind the iPhone in specific. First-person games like Wolfenstein 3D or Duke Nukem 3D or even Prey feel clumsy and unintuitive (Wolf3D, I would say has the best possible control implementation and still feels wrong). On the flip side there are also excellent implementations of the touch screen for some ports. SimCity and Civilization Revolution are excellent iPhone games since with both games the translation of mouse (or cursor) to finger is nearly 1:1.
The game I initially wanted to talk about before going on this tangent was Spider: The Secret of Bryce Manor, which utilizes the touch screen wonderfully. The premise of the game is pretty simple, you're a spider and you need to catch bugs. Most of them (with one exception) need to be caught in a web, which you can make with three or more strands of silk shaped into a triangle or any other closed shape. Different bugs have different behaviors and require different tactics to catch. Some need to be herded, some need to be tackled, and others just require some other level of prediction or trickery to catch. The gameplay is divided into levels consisting of a single area, each generally taking around five minutes to complete, making it perfect as a pick up and play type of game.
The real genius of the game, however, is the implementation of the controls. Touching to the right of the spider makes him move right, to the left he moves left. No clumsy pseudo-buttons or tilting needed. To make him jump you simply flick him in the direction you want him to go. Since he sticks to almost every surface and can jump quite far you generally don't need to worry about missed jumps or falling into a hole or anything. There's nothing at all that can cause instant death and, more importantly, nothing to cause instant frustration.
The web building mechanics are likewise intuitive and just plain fun to make. Some areas may require a bit more precision to form a proper web, especially later in the game, but there's nothing that I would consider an extreme difficulty curve. The most important aspect of the game is that in every way it's enjoyable to play. Once you beat the main adventure mode you unlock three additional game modes, which adds a pretty nice level of replayability to the game.
So there you go, I'm practically gushing over an iPhone game. Who would have thought? At $2.99 the game is a steal (iTunes link) and certainly worth picking up if you have a long trip coming up, or really even if you just want a cool game to play.