Just wanted to give my 2 cents: What you are doing is really dangerous. "In an effort to refine my design skills, I’ve decided each week or so to redesign the landing page of a YC company I dig and whose website I don’t feel so similarly about." Doing a facelift on just the landing page, or rearranging elements around a page is not design. It's a subset of design. What's worse, is that these redesigns you offer gives…
I can see what you are saying here, but at the same time you are proposing something very difficult for this designer. In my past, whenever I have been in a debate over design decisions, there is no objective criticism involved. Even if I've done extensive research beforehand and am citing what I imagine users will do and artistic theory that backs the decisions I made, it can always be disagreed with. "Yeah that might be the case, but it doesn't look as good." or "No, users would do this instead, and want this." Yes, surprisingly that flies in design debates.
I don't think that Kyro should attempt this unless he is going to run A/B tests so that he has actual data to back his reasons. Debate based solely upon opinion are fruitless, never-ending, and are bound to come up all the time in such a public and critical forum as hacker news.
That being said, I don't even think that he should be running A/B tests, and shouldn't need to provide any additional justification. Not only is this a ton of work for something he is doing for personal practice in his free time, but it's often a waste of time. A good designer must be trusted. You hire a designer if you like his work and trust his design decisions, not if you are going to nitpick and tell him to justify and test every shade of color he has used.
I think the most famous case of this happening is with with Douglas Bowman quitting at google to move to twitter as the lead visual designer. Mr. Bowman is quite clearly one of the best design and ux guys out there, and is widely respected. This article provides a lot of insight into his decision, and it came down to them questioning his decisions and asking him to prove everything, which was in the end a waste of time.
http://stopdesign.com/archive/2009/03/20/goodbye-google.html