There are many advantages to having membership in RuneScape.
Firstly, there are some basic playing advantages. One advantage, is that you have access to fullscreen, which gives you more high quality capabilities to see in the game. Secondly, there are no adverts when playing in game.
Secondly, there are many website advantages. One advantage, is that you can vote in polls that are members only. Secondly, you can post on the forums, without having to have the 5 million experience requirement.
Lastly, and most importantly, there are many gameplay advantages. Firstly, there are over 150 more quests available. Secondly, you can explore a 3x larger world. Thirdly, there are about 30 more activities and minigames. Also, there are 9 more skills to train, and gain money with. In addition, you can make your own player-owned house. Plus, there's more armour to wear and fight with. Lastly, there's many more ways to make money.
Although there's many more benefits, these are just a few.