I've only been on Tython so far, the Jedi starter planet, but it all seemed fairly linear without much to do but go from point to point. But then I discovered a datacron at the end of a random cave no one told me to go into, and then I learned that there are more datacrons, and now I'm hooked. I'm finished all the quests on the planet, but I'm not going to be leaving until I find the other two datacrons. It's like a scavenger hunt. I crave more.