I've found the fastest way for me to get through my feeds after an absence is to read them in order from the feed with the least unread items to the feed with the most unread items. It would be really helpful if Newsblur had an option to automatically sort feeds in this order.
Besides # of unread, I'd also like to sort feeds by "date of oldest unread post". Since you automatically mark things as read after a month, I'm often left scrambling to figure out what feeds are closest to expiring. I can approximate this by sorting the river of news, but it's far less efficient and less powerful.