This is a regression from Unity 4.3.4f1. In Unity 4.3.4f1 when the find field in the Hierarchy panel is active, clicking on a Hierarchy panel still leaves find field focused. In Unity 4.5 and 4.6 clicking in the Hierarchy panel removes focus from the find field, the text that was entered in the find field is removed. Instead a default "All" text is written and there is no 'x' at the right end of the text field.

Reproduced with:
Unity Version 4.5.0b2 (b4a039a50231)

Workaround: click on the find field again and the 'x' appears at the right side, so the filter can be cleared. Clicking on the find field and then clicking anywhere but the Hierarchy panel also returns previously in the find field entered text.