While the site isn’t an “official” Microsoft channel, it was created by two MS staffers, so the odds of any useful suggestions getting noticed are a bit higher than (say) whining on a forum. As well as suggesting improvements (I doubt “kill the Ribbon” will get very far, but I’m still hoping the rubbish keyboard shortcut support in Word 2010 might get improved), you can vote on those already submitted. As I write this, the top-ranked idea is to make Word’s HTML rendering better.
MakeOfficeBetter.com is free to use, requires registration to post ideas and vote. To get a better idea of what’s already scheduled for Office 2010, check out video of the forthcoming release. Thanks Angus L!