Jacob Sullum has a detailed discussion in Reason's "Hit & Run" blog of "What Legal Pot in Washington Will Look Like." Jacob compares Washington's I-502 with Colorado's Amendment 64 and notes that while both initiatives legalize marijuana for adults 21 or over, and authorize state-licensed marijuana stories, in other (but not all) ways Washington's law is more restrictive than Colorado's.
Conversely, Washington's law does not allow local jurisdictions to ban marijuana stories within their borders, an option that cities in Colorado will have. If you've followed the Medical Marijuana Update series that Phil has been writing in our newsletter, you'll probably agree that that is a big benefit.
Unlike Colorado's law, which can only be changed by constitutional amendment, I-502 can be amended by the legislature at any time -- with a 2/3 vote for the next two years, or by majority vote after that. Our friend Roger Goodman, election this week for a fourth term in Washington's House of Representatives, told the Seattle Weekly last month that he and Sen. Jeanne Kohl-Welles hope to address the DUI issue, and improve the state's medical marijuana system, perhaps through requiring that impairment be determined by independent experts rather than the per se DUI standard or other means.
Hopefully other changes or expansions to the law will become possible over time too, as voters and legislatures become accustomed to marijuana being legal and are satisfied that things are working. Unlike with medical marijuana, non-patients (over 21) obtaining marijuana will not be an issue anymore in Washington or Colorado.