Many new skills have been added since you've probably been on last. Perhaps 5-10 depending on how long you've been gone. I know it took me 8 hours to barely unlock these skills or get started on them. Runescape 's skill system is what basically creates the game, you won't want to miss out on the new ones if you're going to join. I would take a couple of hours just to explore them for a bit, maybe skill up the ones you take a liking to. My favorite has personally been the new Summoning they have just added. It was always a dream of mine for Runescape Gold to have summoning. 

Various other things have been added besides skills. Obviously, the first thing you will notice is the new “HD” version they have. I can not express my surprise enough at how they can create these amazing visuals in a browser-based game. Other things you might notice are things like the new “Event” description on the server selection. Some servers are dedicated for events such as Tournaments at the Duel Arena and such. There is also another great feature which is called the “Great Exchange” which has greatly increased Runescape's economy. It is similar to an “auction house” of sorts, I'll just leave it at that.