For now, I can just click F1 to manually dim the panel. I'm hoping issues like this will be ironed out after Siri is more widely used, but I wish they had been addressed beforehand. Then I asked Siri to "decrease the screen brightness," and it complied. When I asked it to "lower the screen brightness," Siri told me it could not do that. First of all, Siri sometimes needs you to use specific language.
The two bugs I ran across in Siri both seem like they should be fixed already, and could be fixed by Apple on its server-side end. After I asked it to show me my PDFs (and it did), Siri understood what I meant when I asked to see the ones from the last month. Siri also understands contextual follow-up questions. When I ask Siri to "search the web for photos of cute dogs," not only can I pin those results for later when the Yankee bullpen blows a lead, I can also drag and drop the images onto my desktop. Siri results can be pinned to the Notifications Center sidebar and receive updates, which makes it easy for me to stay up to date on my team's wins and losses.