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 :])
I've always been curious - is it known whether Gabe himself actually reads emails from non-employees sent to that address, or could it just be an assistant that handles routing most of the emails? I really can't imagine how he could possibly have time to deal with a public email account, but maybe I'm overestimating the volume of email he would get...
(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 :])