The core provision of DOMA required the federal government to treat the marriages of same-sex couples one way (as though they had never happened) and the marriages of straight couples a different way (respecting their validity in 1,138 federal contexts). The Supreme Court struck down DOMA both because of that unequal treatment and because the federal government had improperly taken over the states' normal role of deciding who is married and who isn't.
...The demise of DOMA section 3 is also a crucial milestone on the road to LGBT equality because DOMA is the last federal law that requires discrimination against lesbian, gay, and bisexual people.
The ACLU lauds the decision:
Windsor isn't just the death of the core of DOMA, it's the end of official federal discrimination against lesbians and gay men. That's quite something to celebrate!
The Supreme Court yesterday also restored the freedom to marry in California when it dismissed an appeal by backers of Prop 8 on standing grounds. The effect is that the August, 2010 ruling that found Prop 8 unconstitutional will be reinstated.