I don't like any application that doesn't use the system chrome. This includes e.g. WLM and older versions of WMP (where I always turn the chrome on first thing). I also greatly dislike that all of the Silverlight 4 OOB apps I've seen so far use
a custom chrome (I'm not sure if they can have the default chrome or not).
If I used Twitter (which I don't), I would likely not use this app because of its use of a custom window chrome.
For me, it's much more than the aesthetic, an application's window must work the same as the host OS. This includes windowing controls, context menus, and things like Aero Snap. Photoshop is a big violator for breaking Snap. Chrome (the browser) is out because
I hate not having (window) chrome to grab on to. Zune gets partial points for obeying snap, but instead of allowing the OS to show the transparent preview of a Snap operation, it immediately reflows the application. I'd rather have the consistency.