Yes, yes, the Gmail way is to keep everything in one folder and then
use searches to filter your messages. Still, I like being able to
glance at my screen and see 2 personal messages and 3 planner-related
messages.

I use Gnus topics to divide my mail into folders and subfolders.
Mail groups are hidden unless they have mail. Some groups like
mail.misc and mail.planner are generally useful, so I keep them visible
even if they don't have unread mail.

Gnus allows you to automatically score threads and messages up and
down based on various criteria. You can set it to completely hide
boring messages, show them in a different color, show interesting
messages in a different color, etc.

On most mailing lists and newsgroups, I don't bother reading message
bodies. I just scan through subjects, hitting k to kill entire threads
I don't find interesting. Gnus remembers what threads I've killed,
marks them as read, and scores them down automatically. It also scores
up messages containing certain keywords, replies to my posts, and
threads I found interesting.

I put interesting people in my BBDB contact database. Gnus indicates
messages from them with a little + beside their name in the message
summary. If someone I know is interested in a thread, I might find it
interesting as well.

I've set Gnus up to hide quoted text. This makes browsing through
threads much easier because I can concentrate only on the the new
parts. I can hit a few keys to expose sections of the quoted text if
the replies aren't immediately obvious from the context.

I can also set it up to remove ads at the bottom of messages,
particularly long signatures, To: lines with more than N recipients,
that sort of thing. I can tell it to strip out HTML, too.

Sometimes I'll jump into the middle of a thread. I can use ^ to get to
the parent message.

I use swish++ to index and search through my personal and
planner-related mail.

Most of my tasks come in through e-mail. Planner lets me keep track of
my TODOs easily by automatically hyperlinking to the mail message I'm
looking at when I create a task. Dealing with a few items on my TODO
list is much easier than going through a large inbox! =)