This one is important if you rent, or care about people who do. gordonzola has a good summation at his LJ today, but the jist of it is:
Prop. 98 kills all rent control, removes many of California's protections for tenants, repeals environmental rules, and trashes public water projects.
For a more detailed summary with arguments for and against the proposition, Smart Voter has a Proposition 98 page and links to the actual law.