I think "Uncanny Valley" is exactly right. It was something we faced when building an AIR app, we could mimic the look of the each OS pretty closely but it never felt quite right.
In the end we opted to a stylize feel of each OS but not an attempt to recreate the look and feel exactly.
I second that, Uncanny Valley is exactly it. It was the same a few years ago on the web when web apps tried to duplicate native functionality (think filebrowsers etc.), it never worked right. In fact, Dropbox (however much I love it) still has that problem in their web UI.
In the end we opted to a stylize feel of each OS but not an attempt to recreate the look and feel exactly.