Has got to be the adorable icons for weapons and armor!
Like, look at this-
http://wiki.spiralknights.com/images/thumb/9/94/Equipment-Grey_Feather_M...
http://wiki.spiralknights.com/images/thumb/b/b2/Equipment-Striker_icon.p...
I feel like the art team did an AMAZING job on these kinds of little things, that they really characterized the weapons and armors greatly, and that the whole thing just feels fun and good. So Kudos to the art team! I mean, I can forgive them for the new UI and all, its kind of bad looking, but the original art, and the art for all of the new monsters is amazing as well- gorgos and dust bunnies really fit well into spiral knights, as do grave scarabs and blueberries. But those icons are really what defines Spiral Knights to me, you know?
Simple and sweet.
Although it would be nice to see slme unique icons for things like chroma, jelly, gunslinger, blah blah yadda. Anyways, its a nice thing about the game, and a nice thing to do is bring up nice things;
how nice!