When there is a text input next to "start a new discussion," it's reasonable for users to assume that the text input will be used as a section header or something similar. (This is the behavior for InputBox used on WMF wikis.) Users do not expect the text input to be for searching.
The "start a new discussion" link should probably be a proper <input /> button. And the search input should probably be on its own line (put the "start a new discussion" link/button directly next to sort order).
Version: unspecified
Severity: enhancement