Here's what I am excited about as a developer. Access to live service such as skydrive etc. Ability to release the same app on windows phone with minimal changes. =Touch enabled apps in a marketplace where Microsoft does not take a cut as opposed to the 30% cut that Apple takes. What's not to be excited about?
Here's why: No cut to microsoft so cheaper prices of apps
Familiar apps on both my phone and on my tablet/PC. No learning curve and settings/progress/data is shared
Live services integration means my app roams with me to another PC when I buy one.
I am not sure we will see lower prices. Firstly, prices in Apple's store already are, let's say, 'affordable' for most apps.
Secondly, I think many apps are priced to what the market will bear, not to recoup costs plus some profit.
Thirdly, running that store will cost MS money. They are not a charity, so I guess they will try and recover the costs in some way. What that way is, I do not know, but I would guess it would be in adverts, non-free options with better exposure and/or in the cost of Windows.
