just a couple posts ago i listed the different MCE development models. but it left a couple dev models out ... on purpose
DirectX - the current MCE shell seems to be written with Managed DirectX. i actually taught myself Managed DirectX last year to ultimately use it for MCE apps, but that never happened for a couple reasons. first, because WPF is in the works and seems to be a better fit for the apps i wanted to develop. second, because DirectX does not remote to an extender session. now that the XBox 360 is out you basically have to write your app to work with an extender. third, MCML is in the works for full fidelity on an MCE PC and extenders. so i consider DirectX a dead end for MCE development. the only real use i see for it is games directly on an MCE PC. but i don't see that being a market until (if) you can use a wireless XBox 360 controller on a PC. maybe if the console wars weren't in full effect and XBox live arcade didn't rock ...
Web 2.0 - haven't kept it a secret that i absolutely hate the Hosted HTML development model for MCE. so i consider AJAX to be 'putting lipstick on a pig'. it helps, but i don't think it helps enough. WPF beats it by supporting 3D, animations, etc... the only Web 2.0 sites i care about are the ones that offer WS/REST services. then you can build a proper UI for the app no matter if you are running on a 40 inch TV or a smartphone. now i would take Hosted HTML more seriously if MCE came with a built-in web server that would host local pages for MCE devs to share (for lightweight apps) ... don't see that happening. but the real question is if MCML can be served from a web site? that would seriously blow Hosted HTML out of the water, especially on extenders