As an extension of muting channels, you can make any server's channel list a little more compact by hiding muted channels. There are a couple of ways to do this:
1. Mute desired channels
You can mute the channels you want to hide, either by clicking the bell icon in the top-right corner:
... Or by opening up the server's notification settings, accessed here:
Then clicking the bell icons for each channel in a handy, scrolling method:
2. Click the "Hide __ Muted" option in the channel list
Back on the main screen, you'll see the 'Hide __ muted' option (the blank number will change depending on how many channels you muted). Clicking it will fold any muted channels away, leaving just your desired channels:
Important note: the hide muted channels prompt will NOT appear unless you have at least 4 muted channels.