Well, Runescape is a for-profit game developed by a well-funded company. I haven't touched that game since back when it was 2D, so I can't really make a full and honest comparison.
On the plus sides of Planeshift you'll find awesome things like
- You won't get trolled here.

- The game is completely free, 100%. You never have to pay a monthly fee or pay for additional features and you couldn't buy items in-game with real world money even if you wanted to.
- GMs in the in-game chat channels and the owners and developers right here on the forums, reading and responding to your posts!
- An awesome player base ready and willing to help you with learning how to play the game and navigate through the world and its quests.
- The game is being developed and updated, new content comes out quite frequently.
- An intense focus on immersion and RP, creating a deep and interesting world to play in with intricate quests, and real consequences and rewards for your character's actions in quests.
On the down sides of Planeshift, the game is still in its Alpha development, meaning that a lot of features are still in their rudimentary forms and it is mostly incomplete. Keep this in mind as you play around.
The short answer is: if you like roleplaying, this is an excellent place to do so, one of the best on the net if you ask me. If you're more into games where you'll be spending most of your time fighting awesome bosses and grinding out on lowly mobs, then you're less likely to have much fun here. Though you still can, the game isn't very refined in this area.