This is the last year in the Modern category.
2013 brought several UI changes from 2012, such as the inventory, chat, and the leaderboard.
Animations were introduced by then as well.
What's different?
-- Leaderboard -- The leaderboard looks like this. What you can see, is that the leaderboard is more opaque than 2012. For every row on the player list, the background is either white or dark. This is a pattern, black, white, black, and so on. Your big name (at the top) is always there. The font is Arial, and stats are below your name.
Did you know that the leaderboard will stretch to fit anything on it? Good work, Roblox!
-- Chat -- The chat now had fade in and fade out. Chat bubbles lasted longer depending on how much you typed... i think? And that's it.
-- Inventory -- The inventory, (referred as Stuff prior to 2013) now had a transparent gradient on the left and right, and a button with two arrows point upward that would show overflowed items.
-- What I think about it.. (sprinklekar) -- Late 2013 is the best year in my opinion if you're looking at pre modern or old clients. I love the animations, and /e dance being introduced by then was also great.