For example, discussions here already have a pattern of being mostly men, and most people with significant amounts of karma are men - I don't know the data here, but I don't think that's a controversial observation. If you're a woman who is new to HN and trying to explain your work experience as a woman in a relevant thread, right now you can give commenting an honest effort and know that everyone can at least read your words and consider them. But under pending comments, instead women will have to write comments that men approve before those women's comments are even visible.
The pending comments system seems worryingly likely to reinforce HN's existing systemic biases in silent/hidden ways that will be hard to analyze and improve after implementation.
Edit to suggest an improvement instead of just criticism: I've been moderating forums and IRC professionally for six years (for del.icio.us and now for Cydia), and to change the culture here, I'd first try expanding the community guidelines with much more detail and several specific examples of unacceptable comments. It clearly has not been enough to only say "be civil", no name calling, and "no classic flamewar topics" - it's a good start, but vague and incomplete. Expanded guidelines would go along with clearly-indicated removals of unacceptable comments to show that the new guidelines are serious and not just suggestions. I would also try implementing a Metafilter-style flagging system: make the "flag" button consistently visible, with a "pick a reason to flag" menu that has one option per rule category (http://i.imgur.com/Aw03Tl2.png). This serves as an integrated (and "just in time") reminder of the rules, with the bonus of flag counts helping moderators find problem spots.