I've noticed something interesting recently with some of the paid apps appearing in the Windows Phone 7 marketplace. For apps that are essentially just a front-end for publicly available free data, I'm wondering why you would want to pay for them?
For example I chose to publish my Internode Usage app for free (just updated to version 1.1). As I don't have to pay to access the data (Internode make it freely available via a web API) I figure unless the app adds significant additional value then why shouldn't my app be free too?
The same principle applies to the second WP7 app that I've been developing – "Aussie Toilets". It is based on data published by the Australian Government, and as they've already published a free iPhone app, the idea that I would try and charge for basically the same thing on a different platform doesn't seem logical (I'm assuming my app is similar, I haven't actually seen the iPhone one).
You see, for every paid app that uses free data there's a threat looming - another app that uses the same data but which is free. So if a paid app is to be commercially viable then it really needs to stand out – add significant value to the original data (eg. visualisations, interpretations), and stand above any free competition.
There is one reason I can think of for some apps being paid which would otherwise be free. It's related to the fact that developers only get 5 free app submissions per year (but no limit to paid apps), so there is an incentive to publish paid apps (especially considering the penalty for failing app certification for a new free app).
So my challenge to the WP7 developer community – innovate and provide real value, and make the best of an impressive platform. Deliver quality apps, whether they are free or paid. But if you expect me to pay for your app, make sure the value it provides is worth at least the value you are charging for it!