The hex stickers used for category icons are so large they force more white space into the category list. I can only see three categories on my screen. If they were half the size they'd be no less readable, increase the number of categories visible at once, and give more balance to the layout compared to the topic postings on the right. A quick CSS tweak to make the hexes match roughly the personal avatar sizes can do this:
.category-logo {
max-height: 75px;
}
looks a lot nicer (IMHO).
discourse has enough whitespace as it is without more being forced on it by oversize graphics.