One thing that can make a big difference in a client is how well it helps you cope with heavy-traffic lists. Sometimes we need to stay on top of a firehose, and a client that helps filter out the noise and lets you keep track of what's going on in sometimes very long threads is an invaluable tool.
You'll find plenty of opinions about how to do this, but I think it's well established that daily digests are not a great way to deal with traffic. It's great to only check things occasionally, but most list software doesn't let you say when it'll dump the huge pile into your inbox, and most mail software doesn't handle the multipart/digest format very well - usually it just gets combined into one big message.
I think the best way is first to filter the list messages out of the inbox, and only check in on it every so often (more often if it's an important list for your job). You want to be able to quickly skim what's new since you last checked in - new threads, what changed in threads you've looked at already, etc. One way to do this is to have another filter to flag messages that are "interesting" in some way - either to me or mentioning me or my topics of interest, or from interesting people.
I'm going to spread out comments on each client into a few posts over the next few days.