While we have a nice selection of apps on the Marketplace, we can certainly always use some more. And not just random apps but specific ones for certain services. The question is how do we let devs know?
One way is to simply vote. And now you can. Using the third-party service UserVoice, anyone can create a site that allows people to solicit feedback from customers and that's exactly what Scott Dorman, a Microsoft MVP, has done with Windows Phone Marketplace Requests. Folks can go there, type in an app and others can vote, it's really that simple.
Of course what does that mean? Well, it certainly is a way to gauge interest in apps but since this is not a Microsoft site and has no affiliation with the Windows Phone Marketplace, there's nothing really binding here. Furthermore, while many of us would like a Hulu app for Windows Phone, third-party devs can't access proprietary services, so requesting one here may not be the best way to get their attention.
Certainly feedback is good but in the end, there's only one thing that will bring more apps and services to Windows Phone: marketshare. When Microsoft's OS starts picking up steam, devs and companies will notice and respond accordingly. Until then, feel free to vote away.