According to Wiki:
While domestic partners receive most of the benefits of marriage, several differences remain. These differences include, in part:
* Couples seeking domestic partnership must already share a residence, married couples may be married without living together.
* Couples seeking domestic partnership must be 18 or older, minors can be married before the age of 18 with the consent of their parents.
* California permits married couples the option of confidential marriage, there is no equivalent institution for domestic partnerships. In confidential marriages, no witnesses are required and the marriage license is not a matter of public record.
* Married partners of state employees are eligible for the CalPERS long-term care insurance plan, domestic partners are not.
* There is, at least according to one appellate ruling, no equivalent of the Putative Spouse Doctrine for domestic partnerships. 
In addition to these differences specific to state law, should the Defense of Marriage Act be found unconstitutional or repealed, married persons in California might enjoy all the federal benefits of marriage, including Constitutionally-required recognition of their relationships as marriages in the rest of the United States under the Full Faith and Credit Clause.
In addition to these differences specific to the United States, some countries that recognize same-sex marriages performed in California as valid in their own country, (e.g., Israel ), do not recognize same-sex domestic partnerships performed in California.
Many supporters of same-sex marriage also argue that the use of the word marriage itself constitutes a significant social difference, and in the majority opinion of In Re Marriage Cases, the California Supreme Court agreed,  suggesting an analogy with a hypothetical that branded interracial marriages “transracial unions”.