YouTube on Windows Phone 7 is less an app, and more just a shortcut that leads to YouTube’s mobile site, providing a different video watching experience.
The mobile site for YouTube functions well enough, but there is not anything that differentiates it from the standalone apps on other platforms. YouTube is just a scaled back version of the full website with a somewhat more difficult search.
You can sign-in to your account to access your subscriptions, favorite videos, and account. But there is nothing in the Windows Phone 7 app that cannot be found by accessing the mobile site through a browser.
YouTube is a decent app that allows access to the website, but is not dedicated like other platforms.