One of the most important rules of mobile web app development is that you shouldn’t give the user an inferior experience. Wherever possible they should have the same functionality as on a desktop. Where this is not feasible you should at least allow them to switch back to the desktop experience.
Google gets this right on the iPhone, which allows users switch to the desktop version in order to manage labels and filters. However strangely they do not provide this option for the iPad. This is particularly weird as it would be easier to use the desktop version on the iPad than the iPhone.
This kind of inconsistency and lack of attention to detail really frustrates me.