Here's some code that lets you hide groups when they have just a few unread
articles.  You can set the threshold on a per-topic and/or per-group basis.

I use levels to separate my groups by priority but I am subscribed to a lot
of groups so each level contains a lot of groups.  I'm easily distracted so
I wanted a way to hide groups without many unread articles.  That way when
I check for new messages I don't end up reading two or three articles in
each of 25 groups.  Instead I generally end up reading one or two groups
with 25 articles each.

It requires a small patch to gnus-group.el which allows you to specify the
predicate to use in the funcall to gnus-group-prepare-function.  That's the
first attachment.  The second attachment does the rest of the work.

You get a new group parameter called gnus-distraction-threshold.  If a
group has fewer than gnus-distraction-threshold unread articles, it won't
be displayed.  gnus-distraction-threshold defaults to 1 so nothing should
change unless you set the threshold somewhere.  As with any group parameter
you can set it for a topic and have it apply to all groups in the topic.
By default groups with ticked articles are always displayed.

Two key bindings are turned on in the Group buffer:
- C-cd toggles distraction prevention on and off
- C-ct toggles the display of groups with ticked articles

I've only been playing with this for a few days so comments and suggestions
are very welcome!