I think of the marget as segmented into a few tiers. At one end, you have the browser and smartphone market, which would be Flash, iOS, and Android. These involve a small target group of hardware, and typically provide one or few central locations where your content will be placed. (Flash can go anywhere, but there are some major hubs that are a good target for flash games)
Next up is the casual/indie market, which bleeds a little bit into the smartphone market, but in addition adds Xbox Live / XNA, Playstation Network, Unity 3d, and Steam for PC. While Steam is not exactly a platform, it is a fully featured service that feels fairly close to Xbox live in terms of digital distribution and user accounts. These are also singular 'shops' where all associated content for the platform can be found, thus making it easier for someone to find your game. Unity gives you the ability to distribute across a wide range of platforms, including the PC, the browser, iPhone, and Android.
Finally you have the AAA traditional game platforms which consists of PS3, Xbox 360, Wii, and the PC. These are the hardest to target and most likely involve larger budgets and contracts. Also designing a non-casual PC game complicates matters as you have many hardware targets you want your game to run on.