I read my subscription to the harmony-dev mailing list by GMail, and I've configured a filter to automatically mark all such mail with the Harmony-Dev label. However, there can still be a lot of unread mail on a daily basis (since I can't read GMail at work) and often I end up filtering out about 60% of the messages, and then read the remainder.
I start by selecting all unread messages, then unselecting the ones that I want to read, then going through them individually. I can't use 'next' or 'previous' since the label filter for Harmony-Dev contains a mixture of both read and unread messages.
However, it turns out that there's a faster way of just looking at new mail. You can specify "is:unread" in the Search Mail box, and you get just the unread items. So, "is:unread label:Harmony-Dev" will show me all new mail messages in the group that I've not seen; I can then unselect the ones that I don't want to read, and execute the search again. Even better, I can now use 'next' and 'previous' to step through all unread messages.
It's not amazingly obvious you can do that, so I thought I'd share the tip. In fact, there's a bunch of advanced search options. Weirdly, you can't save a search and have it as a virtual mailbox (like the labels are). Perhaps that's a new feature coming in a later beta.