As for services, I likewise noticed that the Pandora app can't stream to the Roku, while it can stream to a Chromecast. Which is also the situation with a Tivo, it shows up as a destination for Netflix and Youtube, but not for Pandora. Which indicates what? Is it the client making the determination or are the destinations not advertising something?
The destination apps have to support it, as well as the receiving device firmware (for discovery/launching when the apps aren't already running in the foreground). The DIAL protocol is open, but thus far neither TiVo nor Roku have published specs for third-party developers to support DIAL - only Netflix and YouTube have been 'blessed' on each platform, through private partnerships.
Also, Chromecast uses DIAL, but it also adds a number of other communication features unique to Chromecast apps that aren't part of the DIAL spec(such as using a time scrubber on a phone to move around in a Netflix video). Not sure if this is true for Pandora, but it's possible the Pandora mobile app may currently only support Chromecast devices and would also need to be updated to support a Pandora DIAL app on devices like Roku or TiVo.