What do you get as a member in Runescape?

I thought of becoming a paid Runescape member, but I'm not sure the expense is worth it. Therefore I'm wondering what are the advantages of becoming a member? What more could I do? I'm already a level 40 free player with I've got 100Kgp and full mith armor.

It's been a long while since I played Runescape but I had a members account at one point so here are the advantages:

• A much larger and more varied world. On a free account you have three pretty similar towns, and the Wilderness. Members get locations like the cave goblin city, the dwarven city etc.
• More powerful items. This is most noticeable for Mages where they have access to more runes, and more robes. Free accounts only have the blue and black wizard robes, members have the mystic robes and more.
• Easier money making. On free accounts you need to do a lot of world hopping to find available resources that aren't swarmed by crowds. The population on members' worlds tends to be lower, and they're spread over a much larger area, so there is less rushing to click first for resources.
• More choices for money making: Stuff like pure essence mining is unavailable to free accounts.
• Further advanced weapons and armour: Free players stop at Rune. Dragon items exist for members.

EDIT

As originally mentioned by Jason Berkhan, there are two other advantages I originally missed.

• Members have a far more quests than free players. Members also get new quests on a regular basis.
• Members have more skills. Skills such as Herblore, Agility and Fletching are members-only.

It's actually not very expensive, as it is a cheaper monthly fee than almost all other online games - I was paying $5 a month when I was a member, though the fee is now $5.95 (CAD).

To add to Macha's answer, the huge number of quests that are available to members was the big selling point for me. They contain a lot of great humour, and many of them contain some great puzzles.

Also, Macha didn't mention the huge number of different skills that are available to members - things like Herblore and Thieving, which give you a larger number of things to grind on.

• Quests: Members get FAR more quests than Free Players. Member quests are also much harder. People regularly complete the hardest free quests at levels as low as 30. Members quests range from extremely easy quests through to some of the most difficult boss fights in the game. Some also include nasty puzzles, like the infamous light puzzle in Mourning's Ends Part II: The Temple of Light. Not only are they harder combatively and in terms of their puzzles, but they are more difficult in terms of their skill requirements.
• Members-only Skills: Members get access to Herblore, Agility, Thieving, Fletching, Slayer, Farming, Construction, Hunter and Summoning. These are all really useful skills that many members couldn't live without. For example, Herblore allows players to make potions mainly to increase their statistics, Hunter allows players to catch specific creatures in special Hunter areas, and Construction allows players to create their own houses, complete with dungeons!
• Extra Content in Other Skills: Every free skill has portions that are only available to members. For example, members can reforge certain types of dragon armour (Smithing), they can create magical staffs (Crafting), they can craft the 'special' types of magic runes i.e. Chaos, Blood, Death (Runecrafting), they can use a FAR greater variety of weapons and armour (Attack, Strength, Defence, Ranged), they can cast a FAR greater variety of magical spells (Magic) and more.
• World: Members get a gigantic world compared to free players. The difference is enormous. Just look at that map. Not only do members get access to all that huge overworld, but they have access to numerous dungeons, minigames, alternate planes and such that many free players haven't ever heard of.

My favorite member's skill is construction. I don't know how many times I've built up and torn down my house, leveling up that skill. It's a grind like anything else, but you have something cool to show for it.