The most critical problem: Hacker News is closed source, right? By any definition that includes Facebook and Twitter, certainly. That'll be a tricky one to let go of.
Personally, it's always drivers etc that I find it difficult to get good open-source options for. Even if you're running Linux the open-source graphics card drivers are pretty terrible, so you take quite a performance hit if you're not prepared to include any proprietary code at all.