Here's one example. On my computer, using reddit in fullscreen means that I have to deal with article titles and comments that span 1000+ horizontal pixels, whereas Digg constrains the width of its comments to 600 pixels wide. The same can be said for Hacker News.
I absolutely cannot stand this because reading text on screen at that width creates a tremendous amount of cognitive friction. Plus, there's no easy solution to this problem. If I snap the window to take up half my screen, the dumb (un-hideable) sidebar ends up eating most of that screen space. That means I have to manually resize my browser window to force a width that makes the site readable to me. I hate this about reddit.
This is a just a drop in the bucket of nitpicks I have with reddit. I use it because it entertains me, but I'm not a fan of its cluttered, low contrast, haphazardly designed UI.