This might sound ridiculous, but Valve are actually really good at responding, or at least reading, emails sent right to the top. If you just send your thoughts to gaben@valvesoftware.com, I'm sure he'll forward it to whoever it's relevant to.
(I say this from experience - I sent him an email saying I thought the end of Half Life 2: Episode 2 was really fantastic stuff, and got a response back from the script writer, who he had forwarded the email to :])